292 wont start after exhaust manifold replacement

[FL71C10]

I took my truck to a mechanic to have the exhaust manifold replaced. It's a 68 c10 with a 292 and 4 speed. No PS or PB. I bought a used manifold on this forum, and brought it to them with gaskets and new studs installed. these guys have been good to me and are honest. The one owner is an old drag racer who still races an old barracuda.

They decided to split the manifolds so as not to disturb the carburetor, and no bolts broke off during disassembly (which was my main fear, and the reason I took it to them in the first place).

I got there friday after work and they had the carb apart apart on the bench checking to see if a float or seat got messed up, as the truck would not start after they got it back together. It was popping and backfiring through the carb.

Then they thought maybe it had jumped time. They took out the #1 spark plug and moved it to TDC. They said the distributor rotor was off by like 40 degrees. They rotated the distributor to line it up just to see if it would start. It didn't seem to make a difference.

I did notice that it backfired and seemed to spit flames behind the carb between the carb and the block. I am wondering if maybe there is a big vacuum leak somewhere.

These engines have timing gears, not chains, right? If so, it wouldn't be very likely to jump time just starting up, would it?

I know they are not supposed to know what kind of timing gear every old motor has, but I was a little surprised they were so perplexed by this problem.

They are talking about pulling the valve cover this morning to see where the valves are at TDC. I'm going to make sure they know that there is no timing chain to stretch.

I am hoping the light of a new day will yield an easy fix this morning. I'm sure they were tired at the end of a long day and a long week on Friday.

Any suggestions would be welcome (except "drop in a new crate engine" or "go LS"?

[Shaky]

You are correct. There's no chain, just gears. Usually, on GM sixes, one gear was metal and one phenolic. I suppose it's possible that a tooth or two came off the phenolic gear or it wore.

How many miles on the engine? Did it run good before this?

I don't think you can do anything until you are sure about the timing. Balancer on TDC, #1 piston on TDC on compression stroke, and distributor properly installed.

I had a 250 that was running rough and when I checked the manifolds were loose! Tightened them up and it ran great. The point being that in my experience they will run with vacuum leaks at the manifolds, but will make noise and not run well.

I'll say this - my '67 292 had the outer ring of the balancer spin on the rubber. On the small chance that happened, you'll never get the timing in...

[Grumpy old man]

steel gear to steel gear , No chain , Why did they have to split the manifolds ? It goes on as one assembly . takes less than 15 minutes to install . Are the newly installed studs to long ? holding the intake away from the block in the center ? if it ran before you pulled the intake/exhaust there is no reason to start playing with the timing to get it to run again ,Now you have to start all over when the problem is just probably the installation ?

[Grumpy old man]

Comes off as one assembly goes back on as one assembly , i'll post some pics

[Grumpy old man]

While they had it apart the passages in the intake (pic on left) should be cleaned out ,they get clogged with carbon,

[Larry Weaver]

Years ago had a 250 that would not like cold starts on humid or cold winter days. Turns out dist was whipped and had all kinds of wobble. Put a rebuilt one in and never had a problem again. Just a thought.

[FL71C10]

Originally Posted by Grumpy old man

steel gear to steel gear , No chain , Why did they have to split the manifolds ? It goes on as one assembly . takes less than 15 minutes to install . Are the newly installed studs to long ? holding the intake away from the block in the center ? if it ran before you pulled the intake/exhaust there is no reason to start playing with the timing to get it to run again ,Now you have to start all over when the problem is just probably the installation ?

I agree. They used the old studs in the head. I replaced the studs where the exhaust pipe hooks up with the donut at the end. The truck has 90,000 miles on it.

[Wrenchbender Ret]

I did notice that it backfired and seemed to spit flames behind the carb between the carb and the block. I am wondering if maybe there is a big vacuum leak somewhere.
[/QUOTE]

You have bad vacuum leaks. The exhaust manifolds on 292 engines are prone to warp when run hard. If useing used manifolds its best to get one from a 250 or 230. The cam gear is aluminum. I worked on fleets of 292,s for many years & never saw a timing gear failure. Don't let them pull the cover.

[FL71C10]

Originally Posted by Wrenchbender Ret

I did notice that it backfired and seemed to spit flames behind the carb between the carb and the block. I am wondering if maybe there is a big vacuum leak somewhere.

You have bad vacuum leaks. The exhaust manifolds on 292 engines are prone to warp when run hard. If using used manifolds its best to get one from a 250 or 230. The cam gear is aluminum. I worked on fleets of 292,s for many years & never saw a timing gear failure. Don't let them pull the cover.[/QUOTE]

It is from a 250. I didn't get a chance to go by there today. I'm going to go by in the morning and see if they figured it out. The fact that they didn't call me tells me they didn't.

I read how used manifolds have crappy castings, are often warped, and don't mate well with the intake, so I figured an OEM one would be better. The whole point was to replace it as one unit. I understand why he wanted to not mess with the carb, but.....

[RichardJ]

>>They decided to split the manifolds so as not to disturb the carburetor<<

Was stupid in many ways. They've screwed up the intake/exhaust gasket. Probably a one-piece gasket. There are also multi piece gaskets.

With a new gasket between intake and exhaust, those bolts should be only snugged up. Bolt the intake and exhaust manifolds to the head and then go back and tighten the bolts between the manifolds.

[FL71C10]

Originally Posted by RichardJ

>>They decided to split the manifolds so as not to disturb the carburetor<<

Was stupid in many ways. They've screwed up the intake/exhaust gasket. Probably a one-piece gasket. There are also multi piece gaskets.

With a new gasket between intake and exhaust, those bolts should be only snugged up. Bolt the intake and exhaust manifolds to the head and then go back and tighten the bolts between the manifolds.

\

I did get a new gasket set that included the one between the intake and the exhaust. I'm pretty sure it was a one piece.

[Jason Banks]

The popping and backfiring through the carb tells me, this has got to be the distributor installed wrong. I know from experience. I can't ever seem to get them stabbed right on the first try. Every time I have put my distributor back in my 250 I have had to stab it multiple times to get it right. They either have it in 180° off or maybe just 1 tooth off.

I don't see any problem with splitting the manifolds. It's pretty easy to check and see if they are straight. Or at least close enough to not cause any HUGE leaks like it would take for it to not run at all.

[FL71C10]

OK, its all fixed. The intake manifolds were apparently not mated properly. They took it apart again and used the new (old) intake, which was what I had intended in the first place. They said the plugs were bad too, which might have made it harder to diagnose.

[randy500]

They killed the plugs trying to start it with the manifold leaks

[FL71C10]

Originally Posted by randy500

They killed the plugs trying to start it with the manifold leaks

Possibly. The truck sat for a lot when the previous owner had it, so the plugs may not have been too good. I think they know they messed up. They didn't charge me very much for the whole job. I don't think they charged me for all of the non-existent timing issues that they spent time hunting down.

I should probably put some new points in it.