B48m will not run right

7690 Views 14 Replies 5 Participants Last post by Mr onetwo, Sep 26, 2011

Mr onetwo

Discussion Starter · Sep 13, 2011

Well guys, I am at the end of my wits...I need your help.

Late last week my 648 died and had to be pushed into the garage.It started by hunting badly and finally needing 3/4 choke to run...then nothing.Engine has spark,compression,ect.It is the original B48M w/ Marvel-Schebler DD11 carb.I thought it was a fuel pump (Napa electric) problem.I took the time to reseal the intake,rebuild carb & install brass float,replace lines and filter,drain tank(no screen) and check timing and points/condenser.All seems good.Installed different pump down by tank.Still won't run without 3/4 choke.Fuel filter is empty???, but when I take off top of carb, fuel bowl is full.I put the outlet of the pump into a can and it pumps real strong and empties tank in about 10 minutes.Could this be a bad coil? Also, the idle and main needle on the carb do not seem to have any effect on engine operation...full in to full out.Governor also seems to not move much and is locked closed when engine is shut off.Is this correct...I can't find any reference to this in any service manual.Any ideas? Thanks

1 - 15 of 15 Posts

boomers_influence

· #2 · Sep 13, 2011

mark
when the engine is off, the governor arm should move,
when the throttle is moved from slow to fast.
thank you. boomer ( the used onan engine parts guy)

99flhr

· #3 · Sep 14, 2011

Mr onetwo said:
Also, the idle and main needle on the carb do not seem to have any effect on engine operation...Any ideas? Thanks

Last time this symptom was posted, the owner had the mixture screws swapped into the wrong holes after the carb rebuild. Don`t know if that that`s possible with a Marvel carb but it`ll only take a minute to check.
Just a shot in the dark

Mr onetwo

Discussion Starter · #4 · Sep 14, 2011

99flhr said:
Mr onetwo said:

Also, the idle and main needle on the carb do not seem to have any effect on engine operation...Any ideas? Thanks

Click to expand...

Last time this symptom was posted, the owner had the mixture screws swapped into the wrong holes after the carb rebuild. Don`t know if that that`s possible with a Marvel carb but it`ll only take a minute to check.
Just a shot in the dark

Yes, I had seen that post so I took a photo and made extra sure that they were not swapped.Any other ideas guys? :think:

99flhr

· #5 · Sep 14, 2011

Mr onetwo said:
Still won't run without 3/4 choke.Could this be a bad coil?

I`m thinking that an already weak spark would be "drowned" by fattening the mixture with 3/4 choke. So I`m thinking fuel delivery. However, I`m a moron and would not proceed without NEW points and condenser as that`s less $ than a coil

99flhr

· #6 · Sep 14, 2011

Mr onetwo said:
Yes, I had seen that post so I took a photo and made extra sure that they were not swapped.Any other ideas guys? :think:

As long as I`m guessing/fishing, does the Marvel carb parts view show ONE of the mixture screws with a little baby o-ring ?

Mr onetwo

Discussion Starter · #7 · Sep 15, 2011

Thanks for the replies and ideas 99 :trink: .I replaced the tiny o-ring when I rebuilt the carb.I now think the both fuel pumps I have are bad....new fuel filter should not be empty when running.I have ordered a Mikuni DF44-227 single outlet vacuum pump off Amazon for $20 shipped.http://www.amazon.com/Mikuni-Fuel-Pump- ... sr=8-1When it comes I hope it is the fix.The stock fuel pump is missing the drive plate and spring so that was not an option to reuse anyway.I will update you when the pump comes. :gums:

99flhr

· #8 · Sep 15, 2011

I`d rig a gravity feed tank or bucket above the carb to confirm or deny feed problems.
One assumes that your gas cap vent is open

gator_rider2

· #9 · Sep 15, 2011

When clean carburetor you need small wire to pass through Idle and main passages they show in venture work back and to brake up blockage then blow out compressed air and nozzle. Get can brake cleaner with straw blow that through passages look in carb. venture to see if brake cleaner passing freely wear safety glass spray always go to eyes.

For note I read that you rebuild carb. wire run through massages I did not read and cleared those passages.

Engine run only when 3/4 choke meaning jets par shall plugged. Re-rebuild carb. common.

Mr onetwo

Discussion Starter · #10 · Sep 15, 2011

The carb was soaked in NAPA carb cleaner bucket overnight and then I blew out the passages and passed a cutting tip cleaner through the passages.I don't think they are plugged.I am going to wait for the pump to come and then hope that the problem was fuel delivery.Has anyone ever disconnected the governor for testing purposes or just run without one? :thumbsup:

Stewart

· #11 · Sep 15, 2011

Mr onetwo said:
The carb was soaked in NAPA carb cleaner bucket overnight and then I blew out the passages and passed a cutting tip cleaner through the passages.I don't think they are plugged.I am going to wait for the pump to come and then hope that the problem was fuel delivery.Has anyone ever disconnected the governor for testing purposes or just run without one? :thumbsup:

Not a good Idea to try running without one :thumbsup: :thumbsup:

It would be a good time to adjust your governor as per the manual. :trink: :trink:

99flhr

· #12 · Sep 15, 2011

[quote="Mr onet.Has anyone ever disconnected the governor for testing purposes [/quote]

Yes, you can disconnect it and "run" the engine with your first digit on the lever

Mr onetwo

Discussion Starter · #13 · Sep 25, 2011

2

Well guys, I finally got the beast to run. :clap: It turns out that there were several factors at play....it illustrates the point of checking everything and returning engine settings to baseline specs.First, both electric pumps I had were no good.Going back to a pulse pump was a good idea.I torn down the motor and found that the valve settings were not correct...closed up very tight.Also, the old head gaskets had been reused by the PO and were leaking a bit.I changed the condenser and reset the points with a light instead of a feeler guage...they were slightly too open.The small brass tube in the throat of the carb had 2 of the holes plugged (Mikuni used to call this the "emulsion tube" in the old days).Also, the PO had tried to remove that tube and had crushed 1 of the 4 holes...I reamed it out.I rigged up an inflation needle with the end cut off so it could go way up inside the carb body and blow it out.2 pcs. of what looks like fuel filter material were forced out.Put everything back together with new gaskets(thanks Gary :thumbsup: ) and after the new pump picked up the fuel it fired and runs fantastic.One thing I found...you must re-torque your head bolts after running awhile...they loosen up a lot.Again thanks for your help! :thumbsup: I hope this is useful :usa:

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99flhr

· #14 · Sep 25, 2011

Glad to hear that your Onan now "abides"
Good work ! Congrats !

Mr onetwo

Discussion Starter · #15 · Sep 26, 2011

99flhr said:
Glad to hear that your Onan now "abides"
Good work ! Congrats !

Thanks for your good ideas and suggestions 99 :thumbsup:

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