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Tunnel ram bogs

12K views 61 replies 11 participants last post by  AutoGear 
#1 ·
I've got a bad bog off idle when nailing it. Tried it at 1100 and 1500 RPM and no difference. Idles at 1100 in gear. 2 x 750 1407 Edelbrocks. Stock jets and springs, accelerator pump in 3rd (top) hole, 3 inch spacers between carbs and breathers/scoop. Timing is by way of a MSD 6AL-2 programmable. 20 degree retard for start, 36 degrees full in at 1000 RPM. MSD pro-billet locked out.

355 CI
11:1
Comp cams XR292R
Duration @ .050 254 intake 260 exhaust
Lift .582 .588
Edelbrock TR1YX tunnel ram 2 x 750 Edelbrocks
Ported double hump 2.02/1.60 3891462 64cc heads
700R4/3500 stall
3.90:1 ford 9"
Race weight 3257
80% Street/ 20% strip car
 
#6 ·
It's fine but you have a problem with it...:rolleyes:

If you think it woke it up,,, Take the tunnel ram off and put one 750 on it and see just how much it will really wake it up... Most motor's like your run it's best with one 750 on it... what does that tell you..
 
#15 ·
Well, all this talk and no advice?? Ok, you need to figure out if its too much air, or too much gas causing the bog. You have a big space to fill. You could try bigger squirters. Was any weight removed from the rear butterflys (the ones that open with vacuum, not the throttle plates)? When you stab the throttle, it bogs, but hit the throttle a few times at that time and see if it takes off or gets worse. If it seems to kind of like it better, it may need a bigger shot of fuel. You may benefit from taking off the spacers - if you need/want the scoop height, put spacers above the carbs to raise it. Might be worth a try.
 
#16 ·
Hey 33, been waiting for your input. Have to order bigger squirters. Thinking that's what it is, more air than gas. No weight removed from the rear butterflys. Hit it, it bogs, let off and hit it again (immediately) and it hauls! I removed the carb spacers from below the carbs (as u advised in another post) and added another 1" spacer above the carb between the carbs and scoop to get the scoop into the wind.
You're right, all this talk and no advise. I feel, if you got nothing to say, then save it.
 
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#21 ·
I need to clarify the timing. It has a 20degree start retard then goes to 36 at 1000 and idles at 1120. Mmmm? It just hit me, in gear it's down around 850-900. But I was at 1100 and 1500 when I tried the 2 lunches (per ur suggestion that I lunch at a lower RPM) and got the bog. The bog seem to go away at around 3500. That is, holding different RPM in park and hammer it (briefly) lol Drivablity is very good, that's what makes it so perplexing. Read the plugs after about 8 miles and was lean so went to 116 mains with 1442 (.063" x .037") Rod and from 107 to 110 secondary, needless to say it still bogs just playing with it in the garage. In respect for the neighbors I shut it down for the night. I removed the spacers the other day and added 1" more between the carbs and scoop to get it back in the wind as you and 33willys suggested.
 
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#22 ·
lock out the programable tming curve. You do not want any timing curve at all.

36deg full time. 36deg at idle that does not move with rpm.
Then reset the carbs idle spee and idle mixture ocne the timing is corrected.
DO NOT JET based on spark plug appearance.
or colour. All you are looking for on the plugs is for excwessive heat and or detonation, NOT Colour.
You jet for power. (Drag strip MPH) Go back to stock default jetting It was not that far off.
You are likely WAY TOO RICH.NOW.
You will not get a clean idle or throttle response until you have corrected the idle timing.
This cam wants locked out full fixed timing. Not a tiiming curve. Eliminate the tiiming curve.

You spark plugs are likely fouled now. Do not use a cold racing plug.

DO not evaluate the throttle response in park. The only throttle response that matters is when driving.
The engine must be warmed up first.
That's what I have, 36 at idle. It has a 20 degree for start then goes to 36 at 1000 and stays there and idles at 1100 in park. It's all done by a MSD 6AL-2 programmable so I can program in any curve or non-curve I need. Using AC 45ts plugs set at .035. I just threw in the playing around in park. Yes, it was warmed up real good.
 
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#27 ·
WOW! A guy sleeps for 6 hours and wakes up to all this great info! I'll do as you say but I can't start until tomorrow. Once I get some of the meaningful information you stated I'll get back to you.
This is gonna be fun! Nothing like pros telling you how to get-r-done! Thanks for all you do!
 
#29 ·
I am going to throw this out - it worked great for me, but you need to be careful. It is all trial and hoping. I had 2 750cfm carters on a tunnel ram, on a 426 wedge. To get it to respond perfect, I took some of the weight of the air doors / butterflies. The transistion did not have any bog at any starting RPM. I launched off an idle (no trans brake) and hit N2O at the same time. It would pull the front wheels (sometimes the left rear too) and never bogged. Car ran low 10's over 138 MPH in the 1/4 on street tires. I had much better response once I took the weight off. This is not for every application - adding weight back on would be a pain, but can be done. I had other motors where I took too much off - you will know if you do - falls on its face.
 
#30 ·
Seems like this would be a very tricky deal. Think I'll keep that as a last resort cause like you said "adding weight would be a pain". I have no way of knowing how much to remove or if it would even help in this case. I'm sure the N2O helped also.
 
#31 ·
Rich 1-3, 2-4, lean 5-7, 6-8

Pulled the new Autolite AR73 .035 gap after the 1st and last ride (about 6 miles) after installing them. 1,3 and 2,4 are showing pretty rich but 5,7 and 6,8 are showing real clean almost like new. No wonder the AFR readings in my post #53 are so screwed up. I didn't find anything wrong with either carb since checking them a week ago other than had to lower float level on the front carb just a hair to get the 7/16 drill bit under it as shown in the carb manual. Didn't find any intake leaks either.
Another question, how much will the plugs clean up after correcting the fuel problem and reinstalling them. Can I depend on the reading from them after that. Good idea to clean them with carb cleaner or ?
The plugs in the pix are lined up as they came out of the engine. 1-7 on top, right to left, 2-8 on bottom, right to left. #1 has been lightly wiped with paper towel before I thought to take the pix.
 

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#32 ·
The sparl plugs show the front carb was clearly flooding.
Dirt in needle seat/fuel pressure/ float setting fuel filter.

Those plugs are not carbon fouled and will not clean up.
You will not be able to further tune , useing these carbon fouled plugs.

Replace all the spark plugs with new. try again.

You are a long way away from playing with the air air door weights.
One thing at a time.
( when if you do get there you will need two spare air doors to work with as it is eaier to get it wrong then right and takes work. adding/subtracting weight.. Don't jump ahead.
Get new plugs and get some more driving afr tests.
Plugs are getting expensive. lol Rather not mess with air door weights if I can help it. Will get back to u after I get things straightened out.
Thanks for all your help.
 
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#42 ·
Sure seems reasonable that DCR's and vacuum plays a major role in tunnel rams.Proper cam selection limiting revision should be critical.While a tunnel ram wants to run in the upper RPM ranges possible the cam doesn't or the reverse could be true.That has to be a good match for it to work.
 
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#43 ·
The WOT afrs look ok. For this one test.
It shows you are in the ball park @ WOT. But drag testing (MPH) will show you what WOT jetting
works best. (The afr gauge is just a guide) It is just one of your tuning tools. There is no perfect WOT AFR to target. Some engines like a bit more or a bit less WOT jetting. Thats what drag strips are for.

Try the new jetting I recomended for the 600's and tell me how it does.
Will get back on it tomorrow and change the jets as u suggested. Not sure when I'll be able to try them though. Gonna be a cool rainy week.
 
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#45 ·
Finally cleared up to where I could get back on it. Reversed the jetting as you suggested from the 95-98 to 98-95, 70x37 needles and 4" springs and reset idle. Been running a vacuum gauge for a while now. Idle vacuum is 9.5-10" and cruise is the same. Here's the latest LM2 logs for your perusal. lol
appreciate all your help and time.
 

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#46 ·
I don't have a 70x42 rod. Got 2 tunning kits. 1 for a 1407 and 1 for a 1406 but no 70x42 rod. lol Go figure! LOL I'll do some math and see how close I can get between the 2 kits. Maybe I can get the rods from summit or ?

Was just checking OD. Usually run round 2500+ RPM when using OD on interstate.
Idle was set with a vacuum gauge to best vacuum 9.5-10". Any hints on setting idle with twin carbs? Have 2 vacuum gauges if needed. Been using one off manifold below carbs. Vacuum is same when using 2 gauges.
Header gaskets are pretty new and show no evidence of leaking. 1600 miles on them.
 
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#47 · (Edited)
Oh yes, always been able to shift the AFR at idle with the IMS screws. Poured marvel mestry oil down carbs at high idle to make smoke looking for an exhuast leak but didn't see any.
AFR looking better? Still looks rich to me at normal cruise and way rich at WOT. I did some math on the jetting and rods looking for as close to setting you suggested last: 95 & 70x37, 98 last log run, u suggested 98 70x42, 95 sec. not having a 70x42 I went with 101, 73x42 and 95 sec. Way i did the math was deduct the rod sizes from the main jet sizes:
95 - 70x37= 25x58 and 98 secondary
98 - 70x37=28x61 and 95 secondary
98 - 70x42=28x56 and 95 secondary
101 - 73x42=28x59 and 95 secondary
Hope this makes sense?
Bout what AFR differance would be in, say, a 3point differance in sizes as in
98 - 70x37=28x61 and 95 secondary (61)
98 - 70x42=28x56 and 95 secondary (56)

Just hit me! I was looking at the rods backasswards! Will try a 101, 73x52 and 95 sec tomorrow.
 
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