backfire thru carb at 3000 RPM

[tiltbilt]

Hi,
New to the forum.
Hope to receive some help on some items.
I am restoring a 1951 Buick Special Model 48
After assembling the straight 8 and running it, I had a backfire thru the carb at 1800 RPM.
So I rechecked my timing chain marks and found I was one tooth off. Corrected that, reassembled, and now no backfire at 1800, but now backfire starts in at 3000 RPM.
I have checked initial timing, mechanical advance, vacuum advance. All are within specs. Tried 2 different carburetors because the backfire seems to go away when you close the choke plate about 3/4 of the way.
Both carbs no difference.
I have checked valve lash several times.
Tried another distributor with no difference.
Do you think I have a wrong or defective new re-ground cam?
Any suggestions would be much appreciated.
Pulling my hair out, and at 63, there is not much left.
Thank You
Tiltbilt

[firstofeight]

Sounds like fuel starvation. Maybe plugged filter. Try using a boat gas can , hooked directly to carb inlet, setting higher than the carb. Or with a low pressure elect pump between the tank and carb. If problem goes away, start chasing the problem back from pump to tank.

Ben

[tiltbilt]

Sounds like fuel starvation. Maybe plugged filter. Try using a boat gas can , hooked directly to carb inlet, setting higher than the carb. Or with a low pressure elect pump between the tank and carb. If problem goes away, start chasing the problem back from pump to tank.

Ben

Thanks for the idea, I will try that tomorrow. But if my memory serves me, I think I checked the carb and it had a full float bowl at the time. (carb has a removable plug to check fuel level.
Has anyone tried running their straight eight at 3000 RPM while sitting to see if this a normal situation?

[TexasJohn55]

Is it doing it no load out of gear or under load while driving?

If choking seems to help, that does sound like a lean condition. Are your manifolds sealed up good?

Have you considered points bounce or too wide? Did you have all your valve springs tested?

I can't seem to get my head around the RPM range changing so drasticly. Valve timing I suppose.

Try to pin it down to one cylinder by pulling plug wires. See if plugs show any color differences.

[tiltbilt]

Is it doing it no load out of gear or under load while driving?

If choking seems to help, that does sound like a lean condition. Are your manifolds sealed up good?

Have you considered points bounce or too wide? Did you have all your valve springs tested?

I can't seem to get my head around the RPM range changing so drasticly. Valve timing I suppose.

Try to pin it down to one cylinder by pulling plug wires. See if plugs show any color differences.

Thanks for the reply,
I am in the process of assembling the car after a frame off, nut and bolt restoration, so I have not driven it yet. It backfires thru carb at 3000 RPM in neutral with no load. I have pulled plug wires one at a time while it is doing it, and cannot isolate it to any one cylinder. Engine has new valves and springs. Have tried adjusting the points tighter and another distributor. No vacuum leaks, had the intake and exhaust manifolds machined together as a unit. I am beginning to wonder if this a normal condition, and maybe wait for a test drive, but would like to know before I install the grill and front bumper in case I have to replace a defective reground cam.

[Jenz38]

The Problem has changed after your new cam setting ? You done the Job like in the manual described ,you know about the 10 teeth differend ? (Sorry for that ,i hope,stupid questions,but only for my understanding now )
I would try another Ign.Coil and condenser if you hadnt done it with the distributor changing.
You have checkt the firing order? If only 2 cyl are interchanged,you can have that Problem.
You checked the dist cap for Cracks ?

[tiltbilt]

The Problem has changed after your new cam setting ? You done the Job like in the manual described ,you know about the 10 teeth differend ? (Sorry for that ,i hope,stupid questions,but only for my understanding now )
I would try another Ign.Coil and condenser if you hadnt done it with the distributor changing.
You have checkt the firing order? If only 2 cyl are interchanged,you can have that Problem.
You checked the dist cap for Cracks ?

All great ideas. Problem did change after new cam setting. Originally backfired at 1800 rpm, now backfires at 3000 rpm.
Set the timing chain as the manual states, also as the picture in the manual shows. (second time). Originally I was one tooth off.

New ign. coil, also tried another condenser.

Checked firing order about 6 times.
New dist cap and rotor.

Has anyone tried to run their straight 8 at 3000 rpm in neutral to see if it backfires(pops)?

[Jenz38]

Another Reason :
sticky lifter ,filled with old burned theavy oil ,witch cant move at higher revs and stop to make it hydraulic.
You can try to give a little more valve lash then zero,like .001. If it starts to get better, you are on the right way.
I think a only regrounded Cam cant be the reason,only then when its wrong inatalled.

[tiltbilt]

OK, Can the shop manual picture be wrong? It shows that there are 10 teeth from the cam gear mark and the crank gear mark. Maybe should show 12 teeth? That would be 10 teeth between the gear marks.
The first time I set it up, I did like the picture shows (10 teeth between gear marks) and it backfired at 1800 rpm. Second time I set it up, I added 1 tooth (11 teeth between gear marks) and it backfired at 3000 rpm.
So now I think I should set it up with 12 teeth between gear marks (that would put 10 teeth BETWEEN the gear marks. Maybe that what the manual means? (to have 10 teeth BETWEEN the marks)?

[Bob]

I would use the chain pins as the count, and 11 looks to be correct.