1970 440 4BBL heat riser, choke and carb spacer questions

[Wing It]

Thank you for reading my post.

I have a 1970 440 4BBL that is a 9.75:1 motor with a slightly larger than stock cam and slightly bigger valves, and running a stock 4BBL. I have seen posts about using a carb spacer to try to avoid the fuel from boiling out of the float bowl. I also have seen posts about blocking the heat riser by using a valley pan/intake gasket with no passage in it. I understand this will render my stock well choke useless. I have already rebuilt the heat riser on the right exhaust manifold and I prefer to have a working choke. The car is a matching number Superbird so I am trying to keep is a original appearing as possible.

My question is this. If I stick with a functioning heat riser and choke but used a carb spacer with my engine specs will I experience boiling fuel or any other similar fuel problems? What thickness carb spacer should be used?

 

[RJRENTON]

If your car is an origional numbers matching Superbird, the carb should be a Carter 4737S manual trans or 4738S auto trans or 4739S manual trans with N95 air grabber. IF this was my car, I'd disable the heat riser valve (just don't install the flapper blade)....that way evetything will look origional, the choke will function just fine, maybe take slightly longer to operate and come off choke, it will reduce or eliminate the fuel percolating issue, because reduction of crossover heat. I have an origional 440+6 car with Holley carbs and using the origional well choke, With the butterfly valve plate in the X-OVER VALVE REMOVED IN THE EXHAUST MANIFOLD, as described, WITHOUT ANY HEAT INSULATING SPACER, and everything operates normally without any percolating in the center carb. Perhaps you should consider a similar method. Just my opinion of course.
BOB RENTON

 

[Wing It]

Bob,

Thanks for your insight. Yes, my car is an automatic car and the carb is a Carter AVS 4738S. I am wondering if the heat riser is working properly (it should as I installed a repair kit) would I not achieve the same result leaving the flapper in tact? When it is hot, and presumably fully open, the resistance to air flow should be similar to it being open with the flapper cut out of it, right?

Thank you for the discussion!

Bob Frey

 

[RJRENTON]

Bob,

Thanks for your insight. Yes, my car is an automatic car and the carb is a Carter AVS 4738S. I am wondering if the heat riser is working properly (it should as I installed a repair kit) would I not achieve the same result leaving the flapper in tact? When it is hot, and presumably fully open, the resistance to air flow should be similar to it being open with the flapper cut out of it, right?

Thank you for the discussion!

Bob Frey

Bob,
Good morning, i trust that you are well and prospering this day. Regarding your question re the heat riser valve, presumably, the heat riser valve, when operating correctly, will not cause any percolation issues. But, if the valve shaft binds up and will not allow full exhaust flow, some of that flow will be directed to the crossover passage in the manifold, promoting fuel percolation in the carb. The issue is: how will you know if the valve is functioning properly? Carb fuel percolation = no; carb fuel not percolating = yes. The caviet is: today's fuels, with a percentage of ethanol blended in, will percolate under ideal conditions, due to the lower Reid vaporization pressure index (measure of volatility). My fix was to eliminate the possibility of a partially restricted heat riser valve was to eliminate the butterfly valve disc, with the external parts intact...for appearance not functionality.....no one knows. I also run the center carb's (primary carb) fuel level slightly below the sight port to reduce the possibility of percolation and booster venturii nozzle drip during a hot soak condition. You could TRY and lowering the float levels, ~ 1/8", to see if that helps. Because of the slightly lower fuel levels, you MAY need to increase the size of the main jets (#120-389, 0.089" dia. to
#120-401, 0.101" diameter). Lower static fuel levels usually cause leaner fuel mixtures due to higher air flow velocity required to start fuel flow.....but this can be controversial. Insulating spacers under the carb can result in warped or breakage of the carb mounting flange, due to uneven tightening of the mounting bolts. As I noted, for me, I just removed the heat riser valve's butterfly disc. I'm sure there will be other opinions and "fixes". Again, just my opinion.....
BOB RENTON