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87 Chevy 366 Bus Engine Problems


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Hey all,

I need some help. I have a 1987 Chevy Medium Duty School Bus with a 366 big block. It has a holley 4 barrel carb and here are the issues:

-I have been experiencing slightly hard starts recently, but it usually does start eventually

-I had to let it idle for a while before driving or it would die quickly

-On shut down (after engine was real warmed up) I experienced long durations of "engine run-on or dieseling"

-I did the SeaFoam down the carb treatment, got tons of smoke out the back and it eventually cleared up and ran good. (this was the last time it ran good).

-Last weekend I couldnt even barely start it. when it finally did start one time I got some backfiring through the carb and the tail pipe and horribly rough idling. It never stabilized and quickly died.

-Today I advanced the timing slightly (not accurately but though it would help). It didnt.

 

What do you guys think is going on? I am really trying to learn how to work on this thing the hard way and not give in to taking her to a shop. Should I just start by replacing the holley carb with a quadrajet 750 and start their?

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Hey all,

I need some help. I have a 1987 Chevy Medium Duty School Bus with a 366 big block. It has a holley 4 barrel carb and here are the issues:

-I have been experiencing slightly hard starts recently, but it usually does start eventually

-I had to let it idle for a while before driving or it would die quickly

-On shut down (after engine was real warmed up) I experienced long durations of "engine run-on or dieseling"

-I did the SeaFoam down the carb treatment, got tons of smoke out the back and it eventually cleared up and ran good. (this was the last time it ran good).

-Last weekend I couldnt even barely start it. when it finally did start one time I got some backfiring through the carb and the tail pipe and horribly rough idling. It never stabilized and quickly died.

-Today I advanced the timing slightly (not accurately but though it would help). It didnt.

 

What do you guys think is going on? I am really trying to learn how to work on this thing the hard way and not give in to taking her to a shop. Should I just start by replacing the holley carb with a quadrajet 750 and start their?

 

You need to clean and rebuild the carb. The run-on is a side effect of high RPMs and a little bit of flooding out( which is what the carb. could be doing). :thumbs: Trust me I've rebuilt so many quadra-jets in my life, and hard starting is a sure knuff sign that the carb needs cleaning or replacing.

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sounds like carb. needs rebuild. be sure it isn't porous. i've seen carbs w lots of miles or age become porous. after running engine shut it off and look down carb throat. to see if gas is seaping into intake... if it is porous sides of venturi stay wet. can cause all sorts of weird symptoms. especially hard starts after it sits awhile. cause all gas has seaped from carb. just another thing to look for.

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I would recommend trying to rebuild the original carb and stick with the present make. If you go to a different carburetor, there may be fuel line routing issues, if the carburetor has a screw in fuel filter, it may not be possible to use it with a different carburetor, etc.

 

I would make sure the float does not leak and sink, which would cause severe flooding. Also, look at the condition of the needle valve, especially its tip and replace if necessary.

 

If the vehicle has an EGR valve, make sure it is not stuck in the open position due to carbon buildup on the stem of the valve. This would cause all kinds of problems, as the valve would be dumping a lot of exhaust into the intake all the time.

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From the sound of things you don't have a lot of experience working on these types of things so I will give you some strange advice.

 

I would tell you to buy a new carb thats like the one you currently have and replace it, then get a rebuild kit and attempt to rebuild the current carb.

 

This would allow you to eliminate the carb as the problem right away and you get a lesson in how to rebuild a carb.

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From the sound of things you don't have a lot of experience working on these types of things so I will give you some strange advice.

 

I would tell you to buy a new carb thats like the one you currently have and replace it, then get a rebuild kit and attempt to rebuild the current carb.

 

This would allow you to eliminate the carb as the problem right away and you get a lesson in how to rebuild a carb.

 

Thanks for your reply and help,

 

I purchased a rebuilt Holley 4152eg which to me is exactly the same as what I had. It should be the best bet for me, not having to do any real modifications to install or run it. It is for a GM 366 engine: http://cgi.ebay.com/ebaymotors/ws/eBayISAP...em=280227269894

 

I'm excited to get this and install it. In the meantime I will change the fuel filter (as soon as I can locate it - I think my manual states its in the frame rail somewhere but isnt very clear).

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Did it originaly come with the holley?

I had an 1990 topkick with the 366 big block, and for the life of me cannot remember seeing a Holley on there. Good engine by the way except for the plug wires they love to eat through. They kept melting from the heat of the manifolds.

Good luck with the new carb

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the saga continues... I installed the new holley 4152 and it truely was identical to the old on. I also installed a new HEI distributor cap and rotor. Still not starting. I replaced the batteries thinking I wasnt getting proper power but still nothing. When you first try to start her, it cranks ok but quickly slows down. I did a spark plug test where I took a plug out and put in back in the boot, held it too the block while another guy tried starting her. I didnt see any spark here. What do you guys think i should look for next? HEI coil? Starter? PS - when i do try to crank it and the cranking slows, I hear a clicking coming from what I thought was the driver side front fire wall. Thanks again for all your help!!!!

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the saga continues... I installed the new holley 4152 and it truely was identical to the old on. I also installed a new HEI distributor cap and rotor. Still not starting. I replaced the batteries thinking I wasnt getting proper power but still nothing. When you first try to start her, it cranks ok but quickly slows down. I did a spark plug test where I took a plug out and put in back in the boot, held it too the block while another guy tried starting her. I didnt see any spark here. What do you guys think i should look for next? HEI coil? Starter? PS - when i do try to crank it and the cranking slows, I hear a clicking coming from what I thought was the driver side front fire wall. Thanks again for all your help!!!!

 

 

Is the timming right? Are there any arks or sparking anywhere around the wires? What about the plug condition? A new coil never hert anything, but I doubt that's it. Try it, though, I guess.

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Inside the cap, look for discoloration around the coil button. Also look for discoloration under the spring that contacts the coil button on the rotor. This is a sign that the spark is grounding out. It was a very common thing to happen on HEI's. The control mod is the thing mounted on the advance plate under the rotor. It has 4 very small wires that the wire breaks inside the insulation. If you have no spark, timing isn't the problem.

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Is the timming right? Are there any arks or sparking anywhere around the wires? What about the plug condition? A new coil never hert anything, but I doubt that's it. Try it, though, I guess.

 

 

 

Timing could be a culprit. No sparks anywhere when cranking. Plug condition is excellent, I replaced them a year ago along with the wires.

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Inside the cap, look for discoloration around the coil button. Also look for discoloration under the spring that contacts the coil button on the rotor. This is a sign that the spark is grounding out. It was a very common thing to happen on HEI's. The control mod is the thing mounted on the advance plate under the rotor. It has 4 very small wires that the wire breaks inside the insulation. If you have no spark, timing isn't the problem.

 

I took a picture of the distributor with Rotor (which is brand new btw) off: What should I be looking for here: It is kinda dirty in their:

4805738961_3e77ae87f8_b.jpg

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Now looking at your distributor, i remember changing the control module (can't for the life of me remember the proper name). Mine had problems starting and running. I do remember it cranking fine however which makes your problem

a little different, but if you have no spark after changing out the important parts I would look here.

You can see it in the distributor picture, and it is at the top and slightly right centre of the distributor

with the two wire plug going into it. Mine was a tbi unit, so it may not pertain to your model, but the parts look similar. I remember the part being cooled by syncing off the distributor, and they are prone to breaking from heat build up, there is a special heat dissipating gel that is applied under the component when installed.

 

Inside the cap, look for discoloration around the coil button. Also look for discoloration under the spring that contacts the coil button on the rotor. This is a sign that the spark is grounding out. It was a very common thing to happen on HEI's. The control mod is the thing mounted on the advance plate under the rotor. It has 4 very small wires that the wire breaks inside the insulation. If you have no spark, timing isn't the problem.

 

I took a picture of the distributor with Rotor (which is brand new btw) off: What should I be looking for here: It is kinda dirty in their:

4805738961_3e77ae87f8_b.jpg

 

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Now looking at your distributor, i remember changing the control module (can't for the life of me remember the proper name). Mine had problems starting and running. I do remember it cranking fine however which makes your problem

a little different, but if you have no spark after changing out the important parts I would look here.

You can see it in the distributor picture, and it is at the top and slightly right centre of the distributor

with the two wire plug going into it. Mine was a tbi unit, so it may not pertain to your model, but the parts look similar. I remember the part being cooled by syncing off the distributor, and they are prone to breaking from heat build up, there is a special heat dissipating gel that is applied under the component when installed.

 

Inside the cap, look for discoloration around the coil button. Also look for discoloration under the spring that contacts the coil button on the rotor. This is a sign that the spark is grounding out. It was a very common thing to happen on HEI's. The control mod is the thing mounted on the advance plate under the rotor. It has 4 very small wires that the wire breaks inside the insulation. If you have no spark, timing isn't the problem.

 

I took a picture of the distributor with Rotor (which is brand new btw) off: What should I be looking for here: It is kinda dirty in their:

4805738961_3e77ae87f8_b.jpg

 

 

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