1997 4.3GS with Holley 4 barrel with flat spot on acceleration at 3500 rpm

dancewithme951

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My new to me 1997 Larson 186 with the Volvo Penta 4.3 GS runs a little rough at idle but more importantly when quickly accelerating over 3500 rpm there is a distinct pause/hesitation befor true acceleration occurs. Throughout the upper rpm range there is ample power and with slow throttle advance there is no hesitation. Also, when cruising at say 4500-5000 rpm I can feel some roughness almost like misfire. Timing is spot on, compression is 170-175 on all 6 cylinders, and I rebuilt/cleaned the carburetor using OEM repair kit 3854341. NOTE: the kit contained a power valve of lower in hg rating (1.5) vs the presumed factory one (5.0) so I went with the new one in case the old one might have been bad. Also new plugs. Lastly, the thermostat might be stuck open. I did a test run today with outside temp of 82 degrees for easily 20 minutes mostly at high throttle and dash gauge temp never got above 120 degrees (laser temp probe on thermostat housing read 110). Any thoughts, suggestions?
 

Scott Danforth

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2 barrel or 4 barrel? or 2300 or 4150/4160?

  • you payed way too much for a $35 holley rebuild kit
  • replace with same power valve that was in there.
  • soak the main body and the metering block(s) in carb cleaner, then chase each passage with a fine wire or fishing line, then blow out with spray can of carb cleaner

when you re-assemble, make sure the idle air bleed slots are centered on the throttle plates

any compression reading over 150psi is a suspect gauge or you have water or oil in the cylinders.
 

dancewithme951

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2 barrel or 4 barrel? or 2300 or 4150/4160?

  • you payed way too much for a $35 holley rebuild kit
  • replace with same power valve that was in there.
  • soak the main body and the metering block(s) in carb cleaner, then chase each passage with a fine wire or fishing line, then blow out with spray can of carb cleaner

when you re-assemble, make sure the idle air bleed slots are centered on the throttle plates

any compression reading over 150psi is a suspect gauge or you have water or oil in the cylinders.
My carb is 4 barrel 4160. And thanks for your responses. BTW: I mis-stated the vacuum rating of the replacement power valve. It was 2.5 in hg, not 1.5. I plan to test the old 5.0 power valve today and may reinstall it if it tests OK.
 

Scott Danforth

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or simply spend $8 on a new power valve. ($11 on amazon if you want it tomorrow)
 

Lou C

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The odd thing is if you look on the Holley website, the carb that they sell as being set up for the OMC/Volvo 4.3 V6s is a 4160 that does not use a power valve but the main jets are bigger to compensate. I know because I have one, but I'm still using the Quadrajet on the engine at this point. Maybe this was changed when GM went to the Vortec heads?

While it doesn't say it on this part of the website, if you look up this carb in the spec data elsewhere there is no power valve and I can tell you having had the bowls off there really is none!
PS, either your thermostat is sticking and not closing all the way or there is a flake of rust or sand in the seat preventing it from closing. Normally that engine should run right at 160, maybe a little lower after getting up on plane and a little higher after coming off plane.
 

dancewithme951

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The odd thing is if you look on the Holley website, the carb that they sell as being set up for the OMC/Volvo 4.3 V6s is a 4160 that does not use a power valve but the main jets are bigger to compensate. I know because I have one, but I'm still using the Quadrajet on the engine at this point. Maybe this was changed when GM went to the Vortec heads?

Whil
The odd thing is if you look on the Holley website, the carb that they sell as being set up for the OMC/Volvo 4.3 V6s is a 4160 that does not use a power valve but the main jets are bigger to compensate. I know because I have one, but I'm still using the Quadrajet on the engine at this point. Maybe this was changed when GM went to the Vortec heads?

While it doesn't say it on this part of the website, if you look up this carb in the spec data elsewhere there is no power valve and I can tell you having had the bowls off there really is none!
PS, either your thermostat is sticking and not closing all the way or there is a flake of rust or sand in the seat preventing it from closing. Normally that engine should run right at 160, maybe a little lower after getting up on plane and a little higher after coming off plane.
Hey Lou C. I appreciate your research. But the cold reality is my carburetor, volvo penta part number stamped on the throttle body is 3857046. The part number was superceded by 3858333. And it does, in fact, have a 5.0 power valve. Also, both the OEM rebuild kit - 3854341 - and Holley renew kit - 703-29 - come with a power valve, the Holley with a 5.0 and the one I used (3854341) with a 2.5 power valve. So, there's no denying that this specific carburetor HAS a power valve - and that apparently per both the Holley and VP/OMC carb kits which both contain power valves - it was designed to have one...just saying. Now, is it possible that an owner of my boat prior to me removed the power valve, threaded a plug in the hole, and increased the jets by 6 sizes? Maybe. Since you state you have the same carb with no power valve but larger jets, what are the size of your jets ( I didn't check the size of mine before reassembling)? And BTW, I tested the 5.0 power valve that was in my carb with a vacuum pump and it would not hold vacuum. I've ordered a replacement 5.0 since I'm still getting hesitation with the 2.5 that came in my kit.

And I'm going to pull the thermostat and test it since some of my symptomotology could be related to an engine running too cool. Thanks again.
e it doesn't say it on this part of the website, if you look up this carb in the spec data elsewhere there is no power valve and I can tell you having had the bowls off there really is none!
PS, either your thermostat is sticking and not closing all the way or there is a flake of rust or sand in the seat preventing it from closing. Normally that engine should run right at 160, maybe a little lower after getting up on plane and a little higher after coming off plane.
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Scott Danforth

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what is the holley list number on the air horn
 

Lou C

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the one I have is:
SAE J1223
80492-1
then under that 2057

If you go onto the Holley site looking for a new carb for the OMC/Volvo 4.3 V6 this is the carb listed but the OP obviously has something different.
 

Lou C

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You know its possible that OMC and Volvo changed the carb # when GM went to the Vortec head design (1996 or so) and Holley never updated their catalog but supplied whichever carb OMC/Volvo switched to. A mystery, because even if you look in an older OMC or Volvo parts catalog you'll find their part number but not Holley's number so no way to tell really.
the primary jets on mine are 68s
on yours they list 63s
so yes you can see without the power valve the jets are larger,
not sure what difference all this makes ultimately.
Scott, thoughts?
 

Scott Danforth

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