12A - Bog under load from 1500-4000 – trying to diagnose
09-19-17, 08:29 AM
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12A - Bog under load from 1500-4000 – trying to diagnose
Recently picked up a ’85 GSL w/5 speed man & 116kmiles. The car has recently had added, a RB inake, RB Holley & RB header with straight pipe to the muffler. We got it and it had a bad bog from 1500-4000rpm if you are going up hill or even just a slight increase in grade. You cannot accelerate past 5 mph or so till you rev it to 4000 and just slip the clutch or dump the clutch and takeoff. It pulls really strong from 4000 & up and it idles great and you can just blip the throttle and it will rev quick and sounds good. Once there is a load on it, it just falls on its face below 4000.
I got it home and figured I would just check for vacuum leaks, check the timing, float level, fuel pressure & see why it’s so cold on the temp gauge.
First thing I did was buy a new thermostat from the local import only parts house to get a good quality one. I put it on but it still acts the same but I have further read that the operating temp is normal at the 1/4 mark. So I am fine with this if that’s normal for these cars.
Next, I got a fuel pressure gauge on it and it was running at 7lbs, I got it adjusted down to 5.5lbs. I also, adjusted the float level with the car running so it just barely comes out the hole. It was way high on both the front & back side. Figured this must have been the issue and it was just flooding it out but the car still drove the same with its bog in the exact same rpm range.
Next, I got the timing light out and found that it had been fooled with but the marks on the pulley were covered in years of grime and not visible so someone had been fooling with it but didn’t have a light. It only had the trailing vacuum advance hoodedup so I disconnected that and have left it unhooked. I found that the Leading and Trailing were swapped from the coils to the dist. cap. I got them switched around got it in time,lined up right with the marks to pointer. I also bought the correct plugs per the RB paperwork, it had some 4 ground electrode plugs and the RB paperwork called for some single ground electrodewith a really thin, pointed electrode. Got all that addressed and figured that had tobe the issue but it now idled at 850 per the spec instead of 1000 as before and it now pulls way stronger from 4000 up but still has the bog in the same 1500-4000 range.
I pulled the carburetor off to see if something inside was doing it. Inside, it still has the 8.5power valve & 49 jets as RB installs. I didn’t see anything inside that would give any issue. There were 2 things I was thinking may be making a vacuum leak, the vacuum advance was loose, and could wiggle side to side and the carburetor base plate and bolts on RB intake manifold didn’t lineup exactly and the carb was in a bind on the studs. I enlarged the holes on the carb base and now it sits down flat with out binding/touching the studs. I put a new carb gasket and buttoned it backup. Fired it up and it runs the samewith a 1500-4000 bog.
Up next, I am getting some plug wires as they look original and I am also getting a cap & rotor. Thinking maybe the plug wires are breaking down. After that I am thinking about pulling the intake and seeing if it is sealing right against the block. I am also curious about the ignition boxes,do they have an idle/low rpm/high rpm circuit? Maybe I have a ignition box going bad. I have read that the power valve in the carb should be blocked off and should jet up to the mid 50's, thinking of doing this but I do not think that would make this big of a bog issue.
Any other ideas on this issue?
I got it home and figured I would just check for vacuum leaks, check the timing, float level, fuel pressure & see why it’s so cold on the temp gauge.
First thing I did was buy a new thermostat from the local import only parts house to get a good quality one. I put it on but it still acts the same but I have further read that the operating temp is normal at the 1/4 mark. So I am fine with this if that’s normal for these cars.
Next, I got a fuel pressure gauge on it and it was running at 7lbs, I got it adjusted down to 5.5lbs. I also, adjusted the float level with the car running so it just barely comes out the hole. It was way high on both the front & back side. Figured this must have been the issue and it was just flooding it out but the car still drove the same with its bog in the exact same rpm range.
Next, I got the timing light out and found that it had been fooled with but the marks on the pulley were covered in years of grime and not visible so someone had been fooling with it but didn’t have a light. It only had the trailing vacuum advance hoodedup so I disconnected that and have left it unhooked. I found that the Leading and Trailing were swapped from the coils to the dist. cap. I got them switched around got it in time,lined up right with the marks to pointer. I also bought the correct plugs per the RB paperwork, it had some 4 ground electrode plugs and the RB paperwork called for some single ground electrodewith a really thin, pointed electrode. Got all that addressed and figured that had tobe the issue but it now idled at 850 per the spec instead of 1000 as before and it now pulls way stronger from 4000 up but still has the bog in the same 1500-4000 range.
I pulled the carburetor off to see if something inside was doing it. Inside, it still has the 8.5power valve & 49 jets as RB installs. I didn’t see anything inside that would give any issue. There were 2 things I was thinking may be making a vacuum leak, the vacuum advance was loose, and could wiggle side to side and the carburetor base plate and bolts on RB intake manifold didn’t lineup exactly and the carb was in a bind on the studs. I enlarged the holes on the carb base and now it sits down flat with out binding/touching the studs. I put a new carb gasket and buttoned it backup. Fired it up and it runs the samewith a 1500-4000 bog.
Up next, I am getting some plug wires as they look original and I am also getting a cap & rotor. Thinking maybe the plug wires are breaking down. After that I am thinking about pulling the intake and seeing if it is sealing right against the block. I am also curious about the ignition boxes,do they have an idle/low rpm/high rpm circuit? Maybe I have a ignition box going bad. I have read that the power valve in the carb should be blocked off and should jet up to the mid 50's, thinking of doing this but I do not think that would make this big of a bog issue.
Any other ideas on this issue?
Last edited by grancuda; 09-19-17 at 08:34 AM.
09-19-17, 10:40 AM
#2
Waffles - hmmm good
iTrader: (1)
Get rid of the holley as its not made to run a rotary very well even from RB.
Get a stock Nikki and intake. Rebuild but don't change the floats or needles
seats unless reallly, really needed. The put it on and see how it runs.
All the things you did are good to do so far. The vacuum advance only being
on trailing is how RB tells folks to install it, no clue why.
Get a stock Nikki and intake. Rebuild but don't change the floats or needles
seats unless reallly, really needed. The put it on and see how it runs.
All the things you did are good to do so far. The vacuum advance only being
on trailing is how RB tells folks to install it, no clue why.
09-28-17, 08:21 AM
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About to get back on this & try tweaking the carb. There are just not any Nikki carbs around this area & I don't want to get a old one off eBay & it have a warped base, wore throttle blade bores. Would a Webber 48 have better drive ability than the Holley?
10-07-17, 06:01 PM
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Finally got it whipped into shape.
Got a set of plug wires, cap & rotor, took for a test drive & that didn't change it any at all.
I then took the CDI boxes off, cleaned the old dialectic grease off, scotch brite'd the mating surface & then plug them back on using dialectic grease on the connections & the mating surface. I also added a ground wire from one of the CDI boxs to the engine block to ensure the distributor is getting a good ground. Took it for a test drive & it ran way better & now would pick up & go with out issue. It now has issues at speed kind of coughing when you stab the throttle.
I added a 1" open block spacer below the Holley, added 58 main jets, took the power valve out & blocked it off. I changed the white accelerator pump cam for a dark maroon pump cam & re-adjusted the lever to be right on the pump for instant response. Took the car out & it ran flawless. All speeds it's responsive, great mid range acceleration & cruising at low RPM it doesn't cough at all if you throttle into it.
Got a set of plug wires, cap & rotor, took for a test drive & that didn't change it any at all.
I then took the CDI boxes off, cleaned the old dialectic grease off, scotch brite'd the mating surface & then plug them back on using dialectic grease on the connections & the mating surface. I also added a ground wire from one of the CDI boxs to the engine block to ensure the distributor is getting a good ground. Took it for a test drive & it ran way better & now would pick up & go with out issue. It now has issues at speed kind of coughing when you stab the throttle.
I added a 1" open block spacer below the Holley, added 58 main jets, took the power valve out & blocked it off. I changed the white accelerator pump cam for a dark maroon pump cam & re-adjusted the lever to be right on the pump for instant response. Took the car out & it ran flawless. All speeds it's responsive, great mid range acceleration & cruising at low RPM it doesn't cough at all if you throttle into it.
