Guest 12lbsofbeans Posted August 12, 2002 Share Posted August 12, 2002 I have a 78 with a fairly built F54/E88 combo 10.5:1 compression .495 lift 282/272 duration cam, head is ported, larger valves, MSD ignition, timing is at 16 degrees initial - goes to 36 all in by 2400 rpm. Fuel is 93 octane 3.5psi seems to hold steady under load. Runs real strong with triple webers. Carbs are 40 DCOE's 36mm chokes 50F6 idles 140 mains 55 pumps with .05 return valve 195 air correctors F16 emulsion tubes With this setup it runs strong, pulls smooth and consistent to 7400 rpm, very little if any hesitation. Low end feels soft, once it hits 3800 rpm it takes off. Its been somewhat down on power though since I moved 2500 ft higher elevation. I was told to run smaller chokes to increase air velocity through the carbs. I tried running 34mm chokes with the exact same jets as listed above; It runs and idles fine, revs fast and strong with no load but take it out on the road and it bogs bad. If you barely give it gas it pulls to probably 3000 rpm then wants to be shifted, if you give it gas it just goes BLAH until you let off then it comes back to life. I thought I'd have to jet leaner with the smaller chokes. But going to 135 mains didn't help seemed to bog worse, so I went to a 40 pump same problem. Well after reading some I swapped in 170 air correctors and this let it rev a little farther before it wanted to be shifted maybe 4500 rpm. Still bogs when you go over 1/ 2 throttle suddenly. I advanced timing from 10 to 16, with the 36 mm chokes it was lean low/mid range but advancing the timing lead to detonation off idle, increasing the idle jet didn't help this is why I was told to switch to smaller chokes - air velocity at lower rpms. Anyway I can run more timing without pinging with the 34's and it feels smoother/stronger I just can't get rid of the bog. Right now its running pretty good. It pulls off idle really smooth as long as you don't give it too much gas. If you floor it, you get a bog then it takes off and pulls harder than before to 7000 rpm. If your rolling partial throttle and try to floor it, it bogs and unless you feather the gas or downshift you have to let off, even if you downshift it bogs then goes this is around 3800 rpm. Under heavy load like climbing hills it starts stumbling feels like its running out of gas. Taking off full throttle its quick and pulls real hard but when you shift at the bottom of the next gear there is a slight bog. The problem seems to be in the transition from partial to full throttle. This is what I'm running right now 34mm chokes 50F6 idles 140 mains 40 pumps with .05 return valve 170 air correctors F11 emulsion tubes Anyone out their with 34mm chokes on a L28 mind telling me what jets your running or maybe point me through my confusion? I thought I'd have to lean it out from the 36's but it seems like the 34's need alot more gas. I'm confused Thanks Quote Link to comment Share on other sites More sharing options...
Mike C Posted August 13, 2002 Share Posted August 13, 2002 Matt, try this post in the carb section as well and check out the "Triples, take me to school" post as it has a lot of info on tuning webers. Quote Link to comment Share on other sites More sharing options...
DAW Posted August 13, 2002 Share Posted August 13, 2002 It sounds more like an accelerator pump problem. It may not be the pump jet sizing but more a problem with the pump system itself; stroke, spring, check ball/seal, etc. I'd double check this system. The bog is happenning when part-throttle is opened rapidly to WOT. It is up to the accelerator pump to cover this transition until the sudden airflow change has mains fuel matched to it. DAW Quote Link to comment Share on other sites More sharing options...
DAW Posted August 13, 2002 Share Posted August 13, 2002 BTW, what size valves (int. and ex.) do you have in the E88? If you're not sure, what yr 240Z is the E88 cyl head from? (This is related to your triple weber question). I think this post could have gone into carburetors forum, but there are a lot of L6 na performance engines with triple sidedrafts..more than any other specific engine type..so why not? DAW Quote Link to comment Share on other sites More sharing options...
DAW Posted August 13, 2002 Share Posted August 13, 2002 BTW, when you have a significant bog/miss/lag with a triple set-up, the problem could be coming from one or two of the carbs and not all three. e.g., if the linkage arm from the main throttle shaft to one of the carbs slips (like from all that crazy throttle action in an autocross)...the car will drive but runs very strange. If you haven't already done this, drive your car briefly without the air cleaners and noting the bog, confirm it's there. Then pull over and check for accelerator pump volume at each carb. You can check only one carb at a time if it's going to be an accurate comparison since the fuel bowl supply is changing, unless you have an electric pump refilling the carbs. You may need a mirror and flashlight to observe the spray but often you can hear and feel the spray discharge and its duration. You're looking for uniformity of function first, then for absolute observation of the pump spray. Hopefully, you'll find one of the carbs has a weaker accelerator pump function than the other two, and problem solved. DAW Quote Link to comment Share on other sites More sharing options...
Guest 12lbsofbeans Posted August 13, 2002 Share Posted August 13, 2002 Thanks DAW, I've made some discoveries since I posted last. One mistake I made in listing what I have; I'm using 50F9 idle jets not f6. My E88 is worked pretty extensively, Malvern racing did part, Sunbelt did the rest I forget which valve size they ended up using but they are larger than the stock 280 valves. I need to find the spec sheet, I've moved 3 times since this motor was built! Since it was running so good with the 36's I didn't think there was anything wrong with my carbs, turns out the first carbs pump rod was sticking/hard to move. Once that was freed up it ran much better. However still lean. Now I'm using 34 chokes 50f9 idles 145 mains 170 airs probably gonna go to a 180 or 185 F11 55 pumps with .5 return So far so good very very small stumble. I'm going to rebuild the carbs in the next month as I think the pump rod and springs need to be replaced and they could use some new gaskets. I did post this in the carb section, response was slow to non existant so I got impatient, sorry. Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted August 13, 2002 Share Posted August 13, 2002 Glad to see that you've solved part of the problem. You may want to consider bumping the idle jets up a size to 55F9. This should clear any of the off-idle stumble you still have. Dave '72 240Z Quote Link to comment Share on other sites More sharing options...
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