After Carb Rebuild motor won't run at low RPM

[green_head_getter]

Hey guys, same 15hp johnson as in my other threads. 15R78B. I have rebuilt the carb on this thing and it ran alot better. However it won't run at low rpm's. I completely disassembled the carburator twice and installed a carb kit. Twice! in a week. The motor still won't run at low rpms. As i gradually start to lower the RPM's and slow the motor down eventually it will hit a spot where it starts to die. Sometimes giving it more gas keeps it going, sometimes more gas kills it off. Everytime it dies it is hard to start and usually won't start unless in WOT.

I can't tell if its not getting gas, or not getting enough gas. Also I turned the carb upside down to make sure my float was sealing the good, i blew into the hose and no air would pass through until i turned it right side up

 

[yesterdaysyouth]

Re: After Carb Rebuild motor won't run at low RPM

what about the fuel pump?? and fuel lines?

 

[green_head_getter]

Re: After Carb Rebuild motor won't run at low RPM

diaphram was bad in the fuel pump but i switched it out with another diaphram. also it doesn't make the motor run any better if i pump the bulb while trying to lower rpm. Same thing. However the bulb is pretty firm but not really really tight. It some give. How do you check to see if the pump is bad?

 

[AlTn]

Re: After Carb Rebuild motor won't run at low RPM

fuel pump test...disconnect fuel line from pump to carb and connect another line from pump to suitable container <I've used a piece of clear vinyl tubing> remove the spark plugs and pull the motor through a few times. You should see a pretty strong and steady pulse of fuel as the pistons move. If this pulse of fuel weakens or even becomes a dribble, your fuel pump is suspect. Are you using champion L77JC4 plugs gapped at .030? What are your compression numbers?. Tested to see if the spark gap will jump 7/16" with an air gap tester? Is the fuel mix fresh? All fuel lines alcohol resistant <new?> and every connection tight so that air can't enter?

 

[mikesea]

Re: After Carb Rebuild motor won't run at low RPM

is the timerbase under the flywheel rotating with the throttle,could be too much timing if the timerbase is sticking,also,is there any fuel coming out the back of the fuelpump flooding the pulse port?no fuel should come out

 

[green_head_getter]

Re: After Carb Rebuild motor won't run at low RPM

is the timerbase under the flywheel rotating with the throttle,could be too much timing if the timerbase is sticking,also,is there any fuel coming out the back of the fuelpump flooding the pulse port?no fuel should come out

Hey guys, haven't changed plugs yet, using what came with the motor. I am going to run your fuel pump test tonight and see if its the culprit. Going to buy new plugs and a fuel pump (if needed) monday to hopefully fix the problem. all the fuel lines look new. no leaking out of the pump. where can I buy a spark tester and a compression tester? Price Range?

 

[AlTn]

Re: After Carb Rebuild motor won't run at low RPM

spark tester..any auto parts place ~ $6....compression gauge with flex hose..~ $16 at harbor freight to a couple of hunnert at snap on...or possibly get at auto zone for a small refundable deposit

 

[green_head_getter]

Re: After Carb Rebuild motor won't run at low RPM

Ok guys i did the pump test and only a few drops and dribbles come out when i pull the motor. I pulled the plugs and they are champion l78V plugs. I am going to buy a new pump and plugs and see if that helps. Also i want to dexarb afterwards. Should i go ahead and buy another carb kit and rebuild my carburator after i decarb? Remember carburatot had a kit put in it 2 days ago. Will the decarb process mess anything up?

 

[green_head_getter]

Re: After Carb Rebuild motor won't run at low RPM

Hey guys boats. Net says that those L78V plugs are what my motor uses. Should i switch to the ones yall suggested?

 

[Sir Robin]

Re: After Carb Rebuild motor won't run at low RPM

The L 78 v is a surface gap plug, suposed to stay adjusted for " life" ???
I removed them from my 1977 35 30 years ago...... L 77j4c's ever since.
better and consistant performance

 

[AlTn]

Re: After Carb Rebuild motor won't run at low RPM

no need at all to fool with the carb after decarbing...might wanna check plugs again after process and at least clean them

 

[green_head_getter]

Re: After Carb Rebuild motor won't run at low RPM

no need at all to fool with the carb after decarbing...might wanna check plugs again after process and at least clean them

ok guys which plugs? i have heard 2 different answers. which is better?

 

[green_head_getter]

Re: After Carb Rebuild motor won't run at low RPM

Bump, curious about these plugs