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Old 06-08-2020, 10:07 PM   #1
RyanAK
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78 Suburban surging then dies on long grade

UPDATE: I thought I had this solved, but it’s back and I’m in the middle of a 1,200 mile trip. The saga continues with post #35. HELP!!

Hey, gang. My daily driver let me down a few weeks ago and put me dead on the side of the interstate 100 miles from home. I’ve methodically gone through diagnosis, fixing or replacing things and THINK I have it solved. I had been able to duplicate the fail a time or two close to home as I worked through my list, but as I completed the last few items I haven’t been able to repeat the condition that left me stranded. BUT... lack of evidence doesn’t mean I have it solved and frankly I’m a little nervous about my next 280 mile commute.

So... the truck: 1978 K10 Suburban pretty bone stock with a GM crate installed by the previous owner. About 18,000 miles on that motor now. Carter Quadrajet, TH350, NP203 (stock), 3.73 gears. The only ‘upgrade’ to the drivetrain would be 16” wagon wheels and 275/75R16 tires.

What happened? Well... Cruising home on I-80 from work near the Delaware Water Gap I hit the LONG grade up into the Poconos at Tannersville. This grade is a grind. Something like 8 miles of 4%. I’m doing 70, keeping up with traffic. Truck is working, but not struggling. Normal. Have done this drive several dozen times. This time however, I get a sudden series of surges and deaths. Sorts like running out of gas. Something like...

La la la going 70 up hill, all is right with the world, la la la.... booooog GO!!! boooooooog GO!! booooooooooog GO! boooooooooog Go. booooooog Go? boooooog go? no? dead. On the berm of 4 lanes at rush hour where two interstates diverge. I have tools and some ability, but I also have two young kids and a wife that like me pretty good. Tow truck.

Get towed to the next exit, Dad drives down to meet me. Fiddle around, get it started. Go for a slice of pizza with Dad. Talk about what we should do. Screw it, let’s try.

Dad chased me home. All good except for one long grade again where it started to do the same booooooog GO!, but not as sever this time. Got it home. Started in on it the next morning.

So... before I go into the long list of relatively small things I did to the truck, I’d love to hear any suggestions for what the cause might be. There are only two things left on my list to try, but as I said, I haven’t been able to replicate the symptoms again.

Thanks in advance. Hopefully I have this licked, but if not, I’m sure someone will suggest the best next move.

Ryan
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Last edited by RyanAK; 07-25-2020 at 11:30 AM.
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Old 06-09-2020, 12:14 AM   #2
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Re: 78 Suburban surging then dies on long grade

By your description it sounds like a fuel starvation issue. I would be checking all the rubber hoses, especially the ones near the fuel tank and maybe above. Could be a cracked hose. Another would be the pump itself starting to go out. I am assuming you have checked the fuel filter in the carb and replaced it
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Old 06-09-2020, 10:53 AM   #3
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Re: 78 Suburban surging then dies on long grade

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By your description it sounds like a fuel starvation issue. I would be checking all the rubber hoses, especially the ones near the fuel tank and maybe above. Could be a cracked hose. Another would be the pump itself starting to go out. I am assuming you have checked the fuel filter in the carb and replaced it
This is a good place to start.

And if you replaced weak or bad hoses you could of easily fixed it.
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Old 06-09-2020, 12:00 PM   #4
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Re: 78 Suburban surging then dies on long grade

Hi, guys.

kw - sorry about cross posting. I don't like it either but I find different guys lurk on the Squarebody board than here in Engine & Drivetrain and this issue really has me a nervous wreck.

Hoses... I checked at the tank after I got towed and all seemed good. Then there's a short section of hose between steel lines on the frame. They seemed good as well. Then at the fuel pump... return like looked good. SUCTION line did not. Extremely soft, tight radius bend so it was sorta flattened. Got the truck started and looked at the fuel pump suction hose again... it was pulsing like an artery. Seriously, it looked like a vein about to burst and was definitely going flat as the pump sucked. When we got the truck home, I replaced this hose with FI line with a longer radius bend. No more flattening, no more sucking closed. I intend to get to all the others to replace soon.

But I was still able to bog the truck out on long grades. It wasn't as bad, but I could still lug it going uphill. Not transmission lug... engine cutting and catching.

I did a bunch of other things while waiting for the right filter to come in at the parts store, none of which eliminated the problem. Ignition tuneup, set timing, replaced a faulty divorced choke. Still bogging on hills.

Fuel filter finally came in. But without a check valve. Whatever... I put it in. And haven't been able to recreate the bogging. But I'm still nervous that I didn't get it. Could this really have been just a dirty filter and soft fuel hose? Were the two enough to starve the Quadrajet when it was really sucking gas?

I'm most nervous about the fuel pump going out. I've had electric pumps go out, but never a mechanical, so I'm unfamiliar with the symptoms or how to check. Do they have a slow death, or catastrophic failure? Fuel flow seemed strong, will checking the pressure reveal the pump's condition?

Appreciate the help. This is my daily driver and my backup vehicle isn't available at the moment, so I'm wound pretty tight about getting this right.
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Old 06-09-2020, 02:44 PM   #5
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Re: 78 Suburban surging then dies on long grade

Mechanical pumps are cheap so I would be inclined to just do it.

Make sure your fuel line is clean. I have had fuel lines that were partially clogged. Also the fuel sock could be deteriorated, and restrict fuel thus causing hesitation up long hills.

If you have 10 foot of fuel line you can bypass the whole steel line to see if that helps.
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Old 06-09-2020, 02:47 PM   #6
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Re: 78 Suburban surging then dies on long grade

As with any fuel line replacement nowadays, whether its FI or carb, make sure the line is rated for Ethanol use....

Also, did you check the fuel filter in the inlet port of the Quadrajet ?

These have been known to go hard and restrict fuel flow under high demand situations...

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Old 06-09-2020, 03:06 PM   #7
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Re: 78 Suburban surging then dies on long grade

I just bought a new mechanical pump. I checked the one on the truck with an not-ideal setup and only got about 1.5 psi and it took about 45 seconds instead of 30 to fill a pint. Could be accurate, could be the wonky hook-up. ('Universal' fitting in the vacuum/pressure gauge from the parts store... sorta shoved into the filter nut...)

$20. We'll put a new one on.

I'll also check for any blocked lines while at it.

The filter in the inlet port of the Quadrajet is what I replaced. That's the only filter with the exception of the sock in the tank. The one i pulled was dark red/brown and hard... so I'm sure it contributed. It took a while for the new one to show up, and since I installed it I haven't been able to replicate the bog-down.

Trying everything before I decide I must go to the tank. I'm a true 'shade tree' mechanic without a garage. Taking the tank down and replacing the sending unit may be something I pay to have done....

Thanks fellas. Feels like we're on the right track.
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Old 06-09-2020, 03:24 PM   #8
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Re: 78 Suburban surging then dies on long grade

I would put a new pump on and add a metal fuel filter in the suction line between the tank and pump.....

Something like a WIX metal canister
https://www.summitracing.com/parts/w...yABEgJCUfD_BwE

I personally dont like plastic fuel filter bodies...have had several split from heat and leak....

As far as the hard red colored inlet filter in the Quadrajet you have found the biggest issue...so many folk either dont know these filters are there, or even worse, they remove them and fill the carb with that red rust material....

New metal can filter, new pump, new inlet filter...dont go getting speeding tickets now!

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Old 06-09-2020, 03:26 PM   #9
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Re: 78 Suburban surging then dies on long grade

Original carb filter had a check valve... new one doesn’t. Any issue with that?
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Old 06-09-2020, 04:55 PM   #10
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Re: 78 Suburban surging then dies on long grade

I think the "check valve" you mention is actually the pressure spring that seats the filter against the carb body....

There are two different length filters, one short at one inch, one long at two inches....both if them have springs that seat the filter in the carb body.

If you still have the "check valve"...could you post a pic so I can be sure

There is usually no "check valve" in the inlet.

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Old 06-09-2020, 10:52 PM   #11
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Re: 78 Suburban surging then dies on long grade

No, I got the spring in the inlet. This is IN the filter. My carb uses the short 1” paper filters. These are available with or without a check valve internal to the filter. There’s a small piston and spring. Not sure what the point of the valve is... but it would certainly restrict fuel flow a bit.



Also... I bogged again. This time I stopped at a convenience store, left the truck running while I ran in for a Coke, and then pulled out and normally accelerated to about 45. Lost power for maybe two seconds, then it caught itself and off we went. No hill, completely flat. Did not accelerate aggressively. Just moseying along. Didn’t happen the rest of the day.

Of note, and possibly related... today and the day I ended up on the side of the interstate have been exceptionally hot. Not sure if that could make symptoms appear or not...
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Old 06-10-2020, 09:17 AM   #12
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Re: 78 Suburban surging then dies on long grade

You wrote that you could see the intake hose pulsing. That tells me that the restriction is further back, probably in the tank. If the fuel pump is strong enough to pulse the hose it is still probably OK. I'd pull the pickup unit from the tank and put a new filter on it.
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Old 06-10-2020, 02:14 PM   #13
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Re: 78 Suburban surging then dies on long grade

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You wrote that you could see the intake hose pulsing. That tells me that the restriction is further back, probably in the tank. If the fuel pump is strong enough to pulse the hose it is still probably OK. I'd pull the pickup unit from the tank and put a new filter on it.
I had forgotten about these check valves...they supposedly stop fuel from draining back out of the carb when the engine is shut off...I never use them...the valve is just one more thing to break....or cause issues.

The issue still seems to me to be more like a total loss of ignition feed for a moment then it comes back and the engine restarts....

Try this....make up a power lead (with a 15 amp fuse inline) and connect the HEI power connection direct to the battery and test drive...remember, you wont be able to shut her down using the key....

If it now doesnt do it, the issue could be in the ignition switch, the plug/s on the back of the switch or the wiring loom itself....( I would suspect the switch or its plug connections)

If it still does it, swap out that HEI control unit and retest...

Double check the ground wires are all in place, especially the one between the cab and the engine...it could be rusty and occasionally going high resistance.
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Old 06-11-2020, 06:09 AM   #14
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Re: 78 Suburban surging then dies on long grade

Does the new fuel pump have good pressure?
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Old 06-11-2020, 03:03 PM   #15
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Re: 78 Suburban surging then dies on long grade

Vince, appreciate the thought. I think the suction line at the pump was it's own restriction - it was very soft and kinked. Seemed to be pulsing on the pump side of the kink. Again, the hose was very soft. I did blow the lines back to the tank and got bubbles on the supply and could hear the return line vent at the filler. So I think the lines are clear. I also soaped everything to try to see if there's a pinhole in any of the lines but since the line wasn't under pressure for very long, it wasn't a very effective test.

Anything to do with the tank... lines right at the tank, the pickup unit, or just a crummy tank... has to wait until I get every other possibility checked. Because this is a Suburban, I can't lift the bed, so it needs to be drained and dropped. And there's the enormous, over-built, welded in, heavy tube steel, 70's era hitch that would need to be completely cut out to drop the tank. It's a really big deal to do it, so I'm hoping one of these other fixes solve the issue. You and everyone else that says 'tank' might be right... but I gotta wait to dive into that. I do appreciate the input. Thank you.

Aussie... tell me about this ground between cab and engine? I've checked most grounds as one maintenance thing or another comes up and all seem good. This is a really clean truck. But... there's one woven ground strap on the passenger side firewall that is just dangling there and I cannot for the life of me figure out where it goes. It's sorta between the heater core and the transmission dipstick tube.

Al, new pump seems to have better pressure. I'm on my way for a test drive and to get a fitting so I can do a better pressure test.

Next steps:

Ignition
Verify 12v at distributor
Jiggle the ignition switch to see if spark cuts out
Check ignition module
Verify vac advance is working (this is on a vacuum temp switch on the thermostat housing... one of the only smog arrangements on this HD emissions truck)
Redo the timing
Follow Aussie's test procedure

Fuel
Re-verify no vac leaks. Are there any mystery spots to check?
Carb - is there a way to clean out any potential crud without tearing the whole thing down and doing a rebuild?
Is there anything that could be altitude related? This was a Montana truck at >3,000' and now we're in Pennsylvania at 542'.

Thanks again fellas. Off to test drive.
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Old 06-14-2020, 02:15 PM   #16
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Re: 78 Suburban surging then dies on long grade

Things got better with the new fuel pump. Also sprayed down the carb really good and dumped a can of Sea Foam in the tank. Seemed to run much better and haven’t been able to reproduce the bog. But it’s been relatively cool here.

Could not locate any vacuum leaks.

I also replaced springs and weights in the distributor and set timing. More improvement. Full voltage at the distributor. Jiggling the ignition switch didn’t cut out spark.

The biggest improvement came when I moved the vac advance to straight manifold vacuum. This truck has a temperature vacuum switch (TVS) that changes the vac advance between ported and manifold based on engine coolant temp. I couldn’t confirm it is working properly, so hooked up to straight manifold vacuum. Seems much more responsive.

If the TVS was wonky and basically eliminated the vacuum to the vac advance can, could that have caused the surging and bogging I experienced?

Still curious if something in the carb could be set for high altitude and have caused my bog out.
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Old 06-14-2020, 02:39 PM   #17
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Re: 78 Suburban surging then dies on long grade

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Originally Posted by RyanAK View Post
Things got better with the new fuel pump. Also sprayed down the carb really good and dumped a can of Sea Foam in the tank. Seemed to run much better and haven’t been able to reproduce the bog. But it’s been relatively cool here.

Could not locate any vacuum leaks.

I also replaced springs and weights in the distributor and set timing. More improvement. Full voltage at the distributor. Jiggling the ignition switch didn’t cut out spark.

The biggest improvement came when I moved the vac advance to straight manifold vacuum. This truck has a temperature vacuum switch (TVS) that changes the vac advance between ported and manifold based on engine coolant temp. I couldn’t confirm it is working properly, so hooked up to straight manifold vacuum. Seems much more responsive.

If the TVS was wonky and basically eliminated the vacuum to the vac advance can, could that have caused the surging and bogging I experienced?

Still curious if something in the carb could be set for high altitude and have caused my bog out.
I know this response will cause a lot of discussion but here it goes...

Plugging your vacuum advance into a direct source will allow it to engage at idle, which is good for a number of reasons. Much like cruise conditions, engines run leaner at idle than they do under load. Again, this means the mixture burns slower and needs an earlier spark to optimize the burn. Ensuring that the mixture has a complete burn before leaving through the exhaust port also helps the engine to run cooler at idle. All carbureted cars were set up with direct vacuum to the distributor before more stringent emissions requirements reared their heads.

Ported vacuum sources are a result of emissions laws and manufacturers doing whatever they could to get big V8 engines to pass smog before the incorporation of the catalytic converter. The idea was that by using little to no spark advance at idle, the exhaust gas would leave the cylinder still-on-fire and help maximize the efficiency of antiquated air injection systems. Engines from this era often ran very, very hot, were prone to warped exhaust valves, cracked cylinder heads and all other manner of issues. Using a ported spark advance will still allow the vacuum advance to do its job at steady cruising, but all of the benefits of idle cooling will be lost.

99% of the time I choose direct vacuum source over a ported source....the TVS should be bypassed unless the vehicle must comply with emissions plumbing rules.

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Old 06-14-2020, 02:40 PM   #18
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Re: 78 Suburban surging then dies on long grade

It's good that you got it resolved. I think the sea foam had its way with the booger in the tank. The other stuff you did probably helped too.
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Old 06-14-2020, 02:58 PM   #19
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Re: 78 Suburban surging then dies on long grade

Great. Thanks guys. It had been a while since I did a Sea Foam treatment.

Aussie - my idle temp has definitely gone down. Just seemed to make everything run better by going directly to manifold vac for the spark.

The way the TVS was plumbed, at ‘normal’ engine coolant temps the vac advance was controlled by ported vacuum. Elevated temps would switch to manifold vacuum. The days I had my stumbles were very warm. The day I got stuck was extremely hot and I was cruising at 70 up a long grade. I’d guess that engine coolant temps were elevated. If the TVS tried to switch the spark to manifold vac and for whatever reason got stuck and eliminated vacuum to the can, could the loss of 15* of cruise timing cause the stumble and surge?

If I sound at all frustrated, I’m not. This stuff is fun.
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Old 06-14-2020, 05:02 PM   #20
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Re: 78 Suburban surging then dies on long grade

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Great. Thanks guys. It had been a while since I did a Sea Foam treatment.

Aussie - my idle temp has definitely gone down. Just seemed to make everything run better by going directly to manifold vac for the spark.

The way the TVS was plumbed, at ‘normal’ engine coolant temps the vac advance was controlled by ported vacuum. Elevated temps would switch to manifold vacuum. The days I had my stumbles were very warm. The day I got stuck was extremely hot and I was cruising at 70 up a long grade. I’d guess that engine coolant temps were elevated. If the TVS tried to switch the spark to manifold vac and for whatever reason got stuck and eliminated vacuum to the can, could the loss of 15* of cruise timing cause the stumble and surge?

If I sound at all frustrated, I’m not. This stuff is fun.
Re "This stuff is fun"...give it time, you are young yet!....haha
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Old 06-15-2020, 12:25 PM   #21
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Re: 78 Suburban surging then dies on long grade

Adjusted the idle fuel/air mix and set the idle. This is a factory A/C truck without a solenoid so the idle gets really low in drive stopped at a light with the air on. Currently researching this. I don't want to increase curb idle to the point where I'm out of the idle circuit in Park with the A/C off. Currently it's at 850rpms in Park, air off. How far could I bump it? I'm still looking to see if a solenoid could be mounted....

Truck is running good. I could probably still fool around with the timing to get things a little more optimum. I did find broken engine-to-firewall grounds and will replace those. Haven't searched for engine-to-frame yet. The battery negative does go directly to the alternator bracket. I've read that without enough grounds that the hard fuel line can act as a ground and get hot, essentially vapor locking the fuel...

Last issue... now with everything tuned up, I have a slight hesitation going into WOT. Just a 1/4 to 1/2 second flat spot before it kicks. Any thoughts? I can live with it as is, but I've taken things this far, it would be nice to get everything tip top.

R
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Old 06-15-2020, 12:51 PM   #22
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Re: 78 Suburban surging then dies on long grade

Depending on your carb origin, you might find a solenoid with bracket here.

https://quadrajetparts.com/quadrajet...c-128_784.html

To find how high you can idle at in park without the mechanical kicking in, just hook up your timing light. Increase or decrease idle speed to see where the timing starts to change.

Sounds like your secondary air door is a touch loose. That causes the bog. It’s adjustable.
Info in here.

https://www.crankshaftcoalition.com/...4MV_carburetor
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Old 06-15-2020, 12:57 PM   #23
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Re: 78 Suburban surging then dies on long grade

Thanks, geezer. I gotta look at the carb itself and see if it has the right holes for a bracket.

I hadn't even thought about the mechanical advance starting to come in... good point and yeah, I can check that with the light. Paperwork with the advance kit says the light springs should start to come in around 1000rpm. I was actually thinking about the carburetor coming out of the idle circuit. Guess I could watch for a vacuum drop with the vac gauge...
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Old 06-15-2020, 06:10 PM   #24
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Re: 78 Suburban surging then dies on long grade

If the condition only shows up after the motor has been running awhile it may be the ignition module. After years of service the dielectric grease breaks down and gets hard. Once this happens the module doesn't use the distributer body as a heat sink anymore and will overheat. You may want to get another Delco module and throw it in the glove box. I had a couple modules go bad that showed the same symptoms along with a coil going bad. The little brush under the coil can be the wrong resistance and cause the coil to overheat but this is usually only a problem with an aftermarket ignition. If the truck acts up and dies on you then will refire after 15 minutes or so and run normal I would look at the module. I also had a carb that would run rich on the secondaries and break up after being floored for awhile. Let off the gas and count to three and it was back to life until you floored it through the gears again.
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Old 06-15-2020, 06:16 PM   #25
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Re: 78 Suburban surging then dies on long grade

I forgot to mention this about the advance curve. The light springs are probably to light for a heavy suburban. I got alot of bounce out of them. Use atleast 1 medium spring. Also don't use the center plate or weights out of a mr gasket
Recurve kit. The factory parts work better. The springs are an experiment and see what works the best. Generally you need heavier springs for a heavy vehicle or light springs in a lighter vehicle. This is not saying that the factory curve isn't to slow for good performance. Its just a general rule of thumb. If you get the really light springs double check the max timing. I have seen them gain a few degrees.
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