Diagnosing a misfire

[lyrikz]

Originally Posted by Mrturner1

Ya I think you're right. I had the 650 dp on the sales counter too and intended to buy it, but the sales guys were pushin for me to buy a 750, and when I made a desicion the two must have gotten mixed up. The speed shop where I got it won't take stuff back unless it's broken or will benefit them somehow. The good news is I am learning a lot about these Holleys and I'm pretty confident I can tune it down to work well with my combo.

Change the carb, or lean it out. Put new plugs in it. Those plugs are bad.

[1969c10joe]

My edelbrock 1405 ran rich the fuel pump pressure was toohigh I installed regulator. It ran so rich the idle mixture screws had noeffect i could close them and engine still ran. After installing reg. I was able to tune carb changing metering rods.You have a carb that is running rich clean floatbowls install fresh fuel filter and then trytuning carb.Big carbs and big exhaust is meant for full throttle high rpm work

[Mrturner1]

Originally Posted by 1969c10joe

you are getting sound advice A 750 carb is big. My truck had a carter 750 i swapped an edelbrock 600 it feels faster. however your carb has as a stuck float or something. ive never had fuel come out of vacuum port. Never owned a holley carb though. Im no carbexpert. Ive had few problems maybe because I change my fuel filters often. I would check floats first

I switched from a perfectly tuned Edelbrock 650 AVS to this Holley 750 dp and it got way faster on the low end and mid/high. With the Edelbrock carb it would spin through first and start to spin in second but bog down and hook, and after the switch it blows the tires off in second as long as you hold the pedal down

[1969c10joe]

We need more info 1did the carb ever run well? if it did then clean and rebuild adjust floats. 2 ha ve you changed anything else? on the carb or fuel delivery system? pump? regulator?? good luck might want to change oil if it has gas in it

[davepl]

Originally Posted by 1969c10joe

good luck might want to change oil if it has gas in it

That's a good point - check your oil to make sure it's not overfilled now (which would indicate a bunch of gasoline has made it past and into the crankcase).

[Mrturner1]

Originally Posted by davepl

Odds are your compression test will be fine then, but still worth doing. You've been washing the cylinder walls for a while with extra gasoline, so it's worth checking just to know.

Remember to disconnect the coil and block the throttle open to get accurate readings.

My big block puts out 160psi, so somewhere in that 140-175 range is probably right. Be better if you had a fresh baseline from when it was new to compare to, but at least now you'll have some kind of baseline (don't throw away the results, because you might want to compare to them in 3 years).

Maybe this will afford me a chance to finally use my cool starter button that I got for setting lash but then found out it's better to hand crank the engine lol I got a Bosch compression checker, hoping that's a decent brand tool. The rotohammers I've used were always great so I figured the Bosch one would be a good tool. I think after work today I'll get a good vacuum gauge too, I keep hearing that guys tune Holleys with one so I better add that to my tool collection. It's funny now that I think about it, my first thread a looong time ago when I was trying to identify what can I had, my tool set consisted of basic box wrenches and a socket set in one of those 100 peice tool kits lol now I have all kinds of pullers and timing lights, every depth and diameter gauge there is, and all kinds of other crap. I never thought I'd end up with all that stuff and know how to use it.

[1969c10joe]

you have quite a ride brought a smile to my face. yeah my 750 was faster top end too not like yours though. looking forward to hearing the solution

[Mrturner1]

Originally Posted by 1969c10joe

We need more info 1did the carb ever run well? if it did then clean and rebuild adjust floats. 2 ha ve you changed anything else? on the carb or fuel delivery system? pump? regulator?? good luck might want to change oil if it has gas in it

It started when I bought a new set of aluminum Ultra Gold full roller rockers because this new solid cam kept on beating my polylocks to the point where the set screw was stripped. I got the roller rockers and a nice set of polylocks to snug everything down but then the misfire started up. I checked lash to rule out the valves not closing all the way which makes the cylinder leak compression, and it's at .015" cold so I think that's where my problem is. The cam card from Comp says to set lash at .020", and bein this is my first solid cam I'm certainly no expert, but .005" less than the recommended lash is probably too tight.

[Mrturner1]

I can just hear GrumpyOldMan now, "how do you like those HIGH PERFORMANCE parts now huh??" lol its comin, just wait

[1969c10joe]

Originally Posted by Mrturner1

It started when I bought a new set of aluminum Ultra Gold full roller rockers because this new solid cam kept on beating my polylocks to the point where the set screw was stripped. I got the roller rockers and a nice set of polylocks to snug everything down but then the misfire started up. I checked lash to rule out the valves not closing all the way which makes the cylinder leak compression, and it's at .015" cold so I think that's where my problem is. The cam card from Comp says to set lash at .020", and bein this is my first solid cam I'm certainly no expert, but .005" less than the recommended lash is probably too tight.

sorry cant help on valvetrain I believe your carb is running rich not due to its size sometimes more than one thing is wrong but clean that carb its free.

[1969c10joe]

been trying to post not working operator error im sure... fuel coming out of vacuum port needs to be addressed maybe more than one thing is wrong but dont adjust valve train to oblivion carb is running rich not because of size

[Grumpy old man]

Originally Posted by Mrturner1

I can just hear GrumpyOldMan now, "how do you like those HIGH PERFORMANCE parts now huh??" lol its comin, just wait

No , I'm not gonna razz you now . It's a good thing we're on opposite sides of the country . We just build trucks in different directions , I start with a bare frame and work up and you start at the front and work back . Only problem is you keep going back ,back,back to the front .

You've saved me money on haircuts because I keep pulling it out reading your engine build threads .... almost weekly .....

I've edited my signature below for your benifit !

[1969c10joe]

I guess i have to keep my post short I have a vacuum guage mounted inside cab. also a vacuum pump is handy for testing under the hood

[1969c10joe]

I am impressed with the amount of work that you have done and hope i dont offend anyone but fuel from vacuum port needs attention maybe a holley expert would help you out. Its not the size of carb

[1969c10joe]

just sittin here waiting for parts the vacuum pump has a gauge on it the vacuumgauge in cab helps me tunemy carb and save gas. torque wrenches are a must. I noticed you invest in tools so just thought id suggest . ive got a cool multimeter has digital tach even a temp probe

[1969c10joe]

I reviewed thread and I would GUESS that when you installed valve train parts you got some debris in carb...as plugs became fouled it ran richer if you change plugs it will run better until they foul too

[Mrturner1]

Originally Posted by 1969c10joe

just sittin here waiting for parts the vacuum pump has a gauge on it the vacuumgauge in cab helps me tunemy carb and save gas. torque wrenches are a must. I noticed you invest in tools so just thought id suggest . ive got a cool multimeter has digital tach even a temp probe

I was actually thinking about putting a vacuum gauge in the cab, but I figured money would be better spent on the one used under the hood to tune. Do you just watch the gauge at different throttle positions and then tune from there?

You're right on about the carb, I put a set of spark plugs in it 2 weeks ago and they looked just like the ones I pulled yesterday after only a few short trips to the store.

These guys have seen me go from not being able to use a timing light to rebuilding a top end on my small block, and there's kind of an ongoing joke about how much unnecessary (and sometimes detrimental lol) wrenching I do on my truck haha I actually REPLACED HEAD GASKETS once about 4 months ago because I thought I blew one, and when I got it all back together it turned out to be one of my spark plug wires wasn't clipped in all the way on the distributor cap lol it's been fun.

[1969c10joe]

you could pop of gauge from under dash plug line then use gauge under hood. I use gauge under dash to select step up springs in edelbrock. I use manual to select rods and jets.BTW the more gas in manifold the bigger the backfire

[1969c10joe]

they wouldnt pick on you if they didnt like you. thats what i was told about 50 years ago

[1969c10joe]

Just thought i wouldmention this fuel in oil can ruin an engine even a small amount may not show up on dipstick. and your plugs? wow pardon me but sure you are not burning oil? Oil creates bluish smokeand fuel moreblack.

[Mrturner1]

Originally Posted by 1969c10joe

Just thought i wouldmention this fuel in oil can ruin an engine even a small amount may not show up on dipstick. and your plugs? wow pardon me but sure you are not burning oil? Oil creates bluish smokeand fuel moreblack.

Yep I'm probably burning a little oil here and there lol I put some decent heads and a big ole solid cam on a stock short block, so it's basically a ticking time bomb but it's a real fun one

[prodjay10]

Anyone ever try using this calculator to figure out which size carb your vehicle needs? Last time I checked, it said my stock 350 cid needs about 340cfm

http://www.carburetion.com/calc.asp

[davepl]

Originally Posted by prodjay10

Anyone ever try using this calculator to figure out which size carb your vehicle needs? Last time I checked, it said my stock 350 cid needs about 340cfm

http://www.carburetion.com/calc.asp

It says 516cfm, which is why a 600 usually feeds them just fine.

A Chevy 350 -just- maxes out a 2GV, becasue adding a 4MV is worth 5hp (250hp vs 255hp). So I figure whatever a 2GV flows, that's what a normal 350 can use.

[Mrturner1]

Originally Posted by prodjay10

Anyone ever try using this calculator to figure out which size carb your vehicle needs? Last time I checked, it said my stock 350 cid needs about 340cfm

http://www.carburetion.com/calc.asp

Also depends what you're using the truck for. 750 is just right when paired with big valves, big intake runners, big intake and exhaust, and 6500 rpm

[1969c10joe]

yes displacement and rpm . It sounds like your truck runs pretty well with 750. with my gearing and sm465 I only got to enjoy it getting on on ramp never did find out when it stop pulling too chicken with those drum brakes