Double pumper carbs and bore wash?

[ian_uk1975]

Guys,

I'm looking to buy an AC Cobra replica with a Ford 351w. The engine spec includes Edelbrock Performer RPM intake and GT40 iron heads. Originally, the engine was supplied with a 600cfm vac secondary carb, but the current owner changed it for a 650cfm double pumper and this has me worried. Some guys on a Cobra replica forum have scared me with stories of premature bore wear with double pumper carbs due to overfuelling and bore wash. I must admit, when I went out in the car, the fumes from the sidepipes were quite heady and the car banged and popped on the overrun. The engine has covered a total of 11k miles, but I'm not sure how many of those were on the DP set-up (guesstimate around 7k-9k miles). There was no blue smoke from the exhaust and the owner claims it doesnt use a drop of oil. Also, the engine did feel very strong when he gunned it.

What do you think?

Cheers,
Ian.

[lovehamr]

I'll take a whack at this.
I think the only time someone washes their cylinder walls with a DP it's because some adjustments are not right. Think about it, with the tuneability of the accelerator pump circuit (shooters, pump cams, pump size etc.) there should be none of that. To put that into perspective, think about all those engines running Webers. If it were just the number of accelerator pumps which were evil then those engines would be toast by the third run. The engine only needs enough fuel during throttle opening to cover the lean condition until the main circuit has responded to the change in throttle opening and subsequent change in manifold vacuum. If the shooters are the right size with the right pump cams, then over fueling won't happen.

Since I can't tell when the smells are coming from the pipes I'll just say that the transient nature of the accelerator pumps usually don't have much to do with an over rich smell from the pipes. For that I would look more at the idle settings and the main circuit. What jets are in the carb?

The "banging and popping on the overrun" are usually due to exhaust leaks which may account for some of the exhaust smell as well.

Just some places to start, Steve

[ian_uk1975]

Should I be worried about this engine? It seems to run well and has plenty of get-up and go, but the UK Cobra club guys have got me worried about the possibility of bore wash. The engine has done 11k miles and dosen't visibily smoke. Oil pressure is 50lb.

Cheers,
Ian.

[Excaliber]

Sounds like it's running a bit rich, eyes burning, smell, etc. I'd rather have a rich condition than a lean one! Running rich is caused by jets that are to big, NOT being a double pumper. Cylinder wash is an extreme condition associated with a misfire on a one or more cylinders when the fuel is not burning. As you indicate the motor runs fine, all though rich, I wouldn't worry about any damage to it. Sounds like 'paranoia' is alive and well, take a chill pill and rejet the carbs, no problem!

That 'over run' condition you mention is; the motor runs on when the key is turned off? Thats often called deiseling as well as 'engine run on'. The NUMBER ONE cause of this is an idle speed set to high. The engine is still rotating when you turn off the key, the engine continues to draw in fuel\air through the carb as a result (slow the idle speed). Hot tiny pieces of carbon or high temperature spots within the combustion chamber ignite this fuel and keep the engine 'running' as a result. Solution? Close the carb throttle plates so the engine 'starves' for fuel\air when shut down. High octane fuel is 'harder to ignite' than low octane fuel and is not as easily affected by the hot carbon in the combustion chamber. THATS why often high octane fuel is another way to stop 'engine run on'.

I would be MUCH more concerned about run on than I would be about cylinder wash. Run on CAN be very damaging to the engine. Like snapping the timing chain for instance.

[NZCOBRA]

Hi an air leak from your header gasket or exhaust will cause poping on overun , lack of cross over (not easy on cobra) and more zig zag baffels down inside of muffler will help remove that de-throttle ugly noise. Get somebody to tune your carb , it shouldn't be that rich , it maybe one or two jet sizes too big or maybe you have your idle air screws too far out, I have had 750 dp on mine for years, there great for that jump on it and go feeling. I would not be worried about having the 650 dp.

[Burgs]

Simple compression, and leak down, tests could be done for a little peace of mind.