Erratic timing at idle

[Maddashimports]

Alright here it goes, I have a 1972 C10 2wd. Just put a brand new motor in it with maybe 350 miles. It's running some worked over 0462 hump heads, 355c.i. with forged internals, Mutha Thumpr cam, 600cfm Edelbrock carb, MSD Street Fire distributor. truck has 10.5:1 compression. First had the initial timing set at 10* to start with. Idled fine in and out of gear. Total advance timing was only up to 24*. Last night I switched out the springs in the distributor with a mr. gasket recurve kit. I installed the black springs which says it should increase the total advance timing the most. Still only 24* total advance and the timing at an idle was bouncing all over the place +-10*, idle was erratic as well, +-300rpm. So.... pulled of the cap, rotor, and made sure the weights were not binding, everything was moving fine. So switched the springs back to the original one. Idled fine last night. Came out this morning and it was doing the same thing now also backfiring out the exhaust at an idle. Think it could be the dizzy? It's a brand new street fire. Chains not stretched either because Im running a gear set. Any input would be greatly appreciated.

[storm9c1]

Odd. You are checking timing with vacuum advance unhooked (and plugged at the carb) right?

That cam probably needs more base timing than 10 degrees. I'd try 12-14 and see if your idle improves. While I've never used one, those who have used Thumpr cams seem to report idle problems.

The last StreetFire I used added too much timing, unlike yours. I had to change the weights to sort that out.

Springs won't change your all-in timing, it will just change how soon (or late) it comes in. The weights and pins on the center beam under the rotor control the max travel. The better distributors come with changeable bushings that slip over the pins to control how far the weights advance. But with a StreetFire, you are stuck with what you got unless you change both the weights and springs.

At this point, I'd recheck your weights and springs. If that checks out, all I can say is to try a different distributor. If you like MSD stuff, the Pro Billet is the way to go. But I've heard a few people report problems with those recently. So I recommend the Chevrolet Performance Distributor. The money you save on a MSD StreetFire is always a crapshoot.

[Maddashimports]

Yeah the vacuum advance is unhooked. I might have to switch to a pro billet dizzy. I've heard people say the Mr. Gasket Co. weights are crap. I'm going to switch back to the msd weights tonight after work. I'm just stumped on the erratic idle and popping out of the exhaust. I've heard of people having problems with new street fire distributors before.

[TJ's Chevy]

Bout the best dizzy out there is DUI. Never had one problem with them. If it is doing with all the original components back in the dizzy then it could be coincidence that something Else is wrong. You could try more initial. Like mentioned, a cam like that could easily use more base. And 24 is not very much for the top. I think mine are at 36. Might try some lighter weights and medium to light springs. Not very skilled on playing with dizzys, but it would make sense to try that out.

[Maddashimports]

Just sent an email to DUI distributors. From what I've read they are about the best. And I've read MSD Street Fire is made in china which it doesn't surprise me its messing up then, hopefully that's what it it.

[geezer#99]

Gear drive, eh!!
What are you using to keep it from walking into the timing cover?

[Maddashimports]

Good question aye. I'll ask the machine shop that built the engine today when they open. I know im going to be swapping it out with a jesel belt drive. Always like the way they look on a vehicle when you pop the hood lol. Plus unlike the gear drive they absorb harmonics well.

[Marv D]

I could think of a lot better place to spend $900 than on a belt drive assembly. Kinda WAY overkill for a street motor and plan on replacing the belt every couple of years at $110. But to each their own.

Just a silly question here, but have you checked the timing with a different timing light to see if things are more stable. I didn't see anywhere that your running a ignition box (with the typical multiple strike below 3000) but it will make timing lights go nutz.

[Boog]

I don't see it mentioned so I'll ask. You have checked voltage to the distributor?

[Maddashimports]

Only reason I wanted a belt drive from jesel is I can get one from a buddy for $350. Plus I've always liked the way they look on an engine. It's like people with dual quads, might not make more power, but sure does look good. Well I pulled out the auto lite 145 plugs that they had in it (.050 gap) and swapped them out with some Accel's I had sitting around. Gapped them at .035, also noticed the accel plugs were about 1/8"-1/4" longer !?!?!? Run's way better and the timing isn't all over the board now. Still switching out the made in china MSD Streetfire with a Dui distributor. Spoke with performance distributor yesterday and it seems like the way to go.

[geezer#99]

Just a fancy looking cover you won't see anyway behind the pump etc!!
Maybe you need a crank trigger instead of that dui!

[Marv D]

YUP! The exposed belt May look kinda cool while going together,,

But once together the belt drive is kinda hid by water pump and everything else. If this is a used setup, be sure and replace the belt. You don't do a many of 8500RPM passes with over 830psi of valve spring pressure to weaken that belt. Yu may thnk it's "OK" for a street motor, but consider the concenquences when it comes apart.

And I have no idea what your talking about on the 2x4's ;-) (this just came out of my 66 pickup bracket truck.

[Maddashimports]

I'll end up switching to a double roller next time I have the engine apart. That is a beautiful engine in the picture btw. Tunnel rams are an exception. I'm mainly talking about setup like the edelbrock dual quad and tri power they sell as kits. Seen quite a few different dyno tests where they don't perform as well as a single carb set up http://www.superchevy.com/how-to/eng...tor-dyno-test/ . The tunnel ram on the other hand is a different monster, they picked up 50hp on the engine dyno on Motor Trend's Engine Masters by switching over to a similar setup, longevity is important. Thanks for the advice, I appreciate it.