Now it's bucking under load...my '64 will never be right!

[nhdoc]

I was so happy that the weber carb conversion went smoothly and the car ('64 220Sb) was actually running great. I discovered the car didn't have any fuel filter so I put an inline filter in and all was fine.

One day the car started bucking, in 3rd gear mostly at higher revs under load. It was hesitating and thumping like a muffled backfire sound. I thought it was fuel starved so I checked the fuel pump and found one of the check valves was bad, so I replaced the fuel pump with a new one and tested it to find it is delivering ample fuel...but still the problem exists. I also replaced the rubber fuel line segment under the car that connects the two steel section of line. I am certain that fuel starvation is not the issue now, at least not from the fuel pump/tank.

Then I opened up the carbs and ensure the needle valves and intake screens were clean and the jets were not clogged...they were fine but I blew everything out anyway. I triple checked the float height which was within 1mm of spec. Still, it bucks and sputters.

I'd guess it starts at about 3000 RPM and usually not until 3rd gear. Tomorrow I will take my timing light to it and check the timing and advance to see if that is causing it...but if anyone has any ideas I'd be open to suggestions.

[Mike D]

Time to break out the Unisyn. Run the engine to about 2500 rpms and check the synchronization.

With the engine not running, open the throttle to half travel and check to see if both carbs are open the same amount. Check for worn linkage bushings.

Lordy, ya gotta love multiple carburetion!

[nhdoc]

I hadn't considered carb sync as an issue.

I did notice the hold down bolts were not as tight as they were when I installed them and it was too hot to tighten them after driving it so I will do that tomorrow am.

I also noticed the vacuum port to the advance had almost no vacuum, hoping that is from the loose bolts and tightening them down will restore vacuum and solve the bucking bronco.

[t walgamuth]

When I read your title I thought you said buckling!

this is a lot better problem to have.

sounds like a vac leak.

Tom W

[sjefke]

I agree with Tom. Sounds like a vacuum leak. Been there done that with SU carbs, and even a Weber 40DCOE. The loose bolts/nuts are key indicator. Get some good locking nuts/rings. You may have to add some liquid gasket for proper sealing. I don't think carb sync can cause such severe bucking. Unless a throttle linkage is loose, or one carb completely non-functional, you would just get a rough running engine (assuming manifold is open to both carbs).

Good luck,

Bert

[nhdoc]

Originally Posted by sjefke

I agree with Tom. Sounds like a vacuum leak. Been there done that with SU carbs, and even a Weber 40DCOE. The loose bolts/nuts are key indicator. Get some good locking nuts/rings. You may have to add some liquid gasket for proper sealing. I don't think carb sync can cause such severe bucking. Unless a throttle linkage is loose, or one carb completely non-functional, you would just get a rough running engine (assuming manifold is open to both carbs).

Good luck,

Bert

All good advice and on today's agenda...thanks. I feel confident that I am on the right track here.

[nhdoc]

Well, the vacuum leak is what caused the problem. Tightened the hold down bolts and it is fine now...except my centrifical advance is not working...but I started another thread for that one. If the bolts loosen again I will put some lock washers on them but I suspect they just got loose because the spacers/gaskets settled a little bit.

[Mike D]

Nothing like a nice easy little fix to make the day brighter!