ill start off with brief run down 31 ~rpu, 350cid .040 over 10-1 comp "461" double humps (port work) 2.02/1.06 cam: 230/236@ .050 .490/.490 110* TH 350 2200-2400 stall 2.79 gears wieand hi ram single 4 here is the problem, i have the common off idle "bog". i have nearly tuned my way thru this issue with a 1to 1 linkage, a four corner idle set up and a 2" carb spacer BUT it runs terrible with any air cleaner set up , will barely pull thru the stumble with a filter !!! take the filter off and it tears a$$ all the way past 6k. any input would be helpfull
What carb are you running? sounds like a squrter problem. If its holley change the nossle size. The carb sounds like its on the rich side and using a filter just makes it richer. What do the plugs look like? What filter are you using? Im running a 400 sbc, eldelbrock perf RPM hyd roller, same tunnleram with a 830 holly db. I put mine back to stock jettings 78 square. Though my airfilter doesnt affect my performance (cheepy spectrer 9 X2 in filter). I did notice it would affect idle once I put it on and would have to compensate for it.
750 holley , 37 squirters and 70 jets "square" and a 14" x 3" paper element!! how about your secondary opening speed?? did u leave it stock?? or speed it up
The 70 jets are "small", depending on the model number it came with somewhere between 70-72 jets with a "65" power valve in the front and 80-82 jets in the rear with no power valve. If you installed a power valve in the rear you will be 8 -10 steps smaller than without the power valve. Check the number on the front of the choke stack and compare it to the chart in the link for the factory jet and squirter sizes, you shouldn't have to "jet down" from stock, doing so will cause a very lean condition. The 2" spacer shouldn't be helping the situation either, the plenum area is already "big" and the velocity needs to be up to keep the response smooth, the spacer is just adding volume and adding to the problem, it is not helping distribution either. http://www.holley.com/data/TechService/Technical/Carb Numerical Listing.pdf
power valves front and rear. i have had the same thought about the 2" spacer. but the choke horn has been removed i went down on the jets due to the 1to1 linkage.... but im listening to all sugestions. thanks guys!!
The linkage shouldn't change your jetting. What power valves are you running, I assume the carb is "relitively new" since it has 4 corner idle adjustment. Is the "bog" occuring during driving or just free revving and is it a "stumble" or does it seem like the engine is just shutting off ? . . . sent you a pm
actually i put a front metering plate off another 750 on the back of mine! i free revs like crazy (prbly too good) the bog is when you stab it from a stand still below 5mph , it just holds back, just wont rev past 2500 for about 100ft, and then starts to breath thanks may take u up on the offer (PM)
. . . so this was originally a "double pumper" without adjustable secondaries ? You won't get idle mixture adjustability on the secondary side by just putting a metering block with adjustable idle mixtures on the secondary side of the carb. You would have to go into the base plate and block off the idle slot to separate the primaries from the secondaries and if I remember correctly you also have to drill a couple holes in the main body. The bog is probably just a result of the tight convertor (that's alot of duration at .050 for that convertor) and no rear gear, I would take the 1-1 link off and try going down on the squirters to 31's and see if that helps.
sorry on the cam specs duration at .050 is 230/236. and i didnt know about the other mods on the carb to properly convert to a four corner idle.. but! i dont know exactly whats happening but when i adjust the screws on the new metering plate i "change the run".. ??????
Just me but I had no luck with power valves with mine. I like the idea about the larger squirters. Also tunnel rams like a relatively high idle speed say 900-1100 rpms. That could help a tad too.
I agree with paulie, your combination is all wrong. Too much duration for the convertor and gearing your using. Did you buy the carb new and modify it yourself or was it used and your not sure what all has been done to it? Get rid of the spacer, the signal to the carb is already weak at low rpm on a tunnel ram and the spacer just makes it worse. A stock Holley list#4779 750 double pumper is probably your best choice for your setup. Advancing your cam 4 degrees will help but at some point you need to address your rear gear issue.
What size accelerator pump are you using? I went with a 50cc pumps, black pump cams, #35 squirters, 3.5 power valves, and got rid of the bog/stumble. Make sure you're running enough advance in your timing. Tunnel rams like more. I agree with others that your rear gearing is part of the issue. Check visually to see what kind of spray you're getting from your squirters when you work the throttle too! I recently had my stumble return and found my backcheck in the accelerator pump had warpage in the rubber and wasn't sealing, so it pushed gas back into the bowl instead of giving a good shot. Replaced them and it's back to working great.
I would definitley investigate what 1971BB427...and others have to say, but also the gears are not helping.....does it rev free in nuetral? is it only bogging under a load?
4:11's and a converter will likely help it out...a lot, maybe upgrade ignition as well...what are you running for an ignition system
a solid linkage from the primary shaft to the secondaries, so they open at the same time 1to1 mine is adjustable i made it from stianless #8 screws and nuts welded together
hey 1971bb427 what is your vaccume reading , to make u chose "3 5" power valves mine runs 14inches in park, but i know u are supposed to check it in gear all i had available was 6 5 power valves
its 1to 1, yet completly adjustable back to stock. maby i should have said that!! "as your post count goes up"
My first thought was converter/gear issue. Even with a more loose converter that gear is just too high. Safely jack the rear wheels off the ground and run it in gear. Bet the "bog" goes away. peace
Agreed - it may not be bogging at all. You are slamming up against a pretty low stall speed, and your cam is struggling to make any torque at that RPM. Meanwhile the 2.79 gears mean you need a ton of torque at that RPM to get any acceleration going. The tunnel is badly hurting the low end torque too. Even without the 1:1 linkage, if you stomp the throttle from a low speed roll as described you are going to open all four venturis at once. If you want a tunnel ram that will make gobs of low end torque and work with your 2.79 gears, google "Real Street Ram".
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