bogging under full throttle

Discussion in 'General Questions' started by Timothy lucas, Mar 24, 2016.

  1. Timothy lucas

    Timothy lucas New Member

    Ok I have an 80cc chinese engine. It was running fine but something happened when I think I mixed the wrong oil with the gas. I quickly corrected it and also added a boost bottle set as well as polished the exhaust ports and roughed the intake port. Now it's still bogging under full throttle unless I hold the clutch a little and allow it to rev way up. Then it gradually speeds up and I can let go of the clutch and get on with it.
    I have vollied between two carburetors as well as played with the air to fuel mixture. I don't know what else to do. Help

  2. Wolfshoes

    Wolfshoes Member

    One bike I have, had a similar problem since late last summer up until today in fact. The bike has a 2011 batch Grubee Skyhawk 66 HT engine with a factory CNS YD EPA Carb with the three plastic air tube vents. The bike would start and idle fine. Turn the throttle a little bit and it was like the spark plug stopped firing. Down hill the engine would run at higher rpms. The engine would rev up at a stop like there was a manifold vacuum leak. I glued the carb on which did not help. Swapping out the complete ignition system did not help. Taking apart and checking the carb twice did not help. Taking a used bike inner tube, I cut off a section having the fill nozzle, glued and clamped one end shut and clamped the other end to the carb intake so a test could be done to detect manifold vacuum leaks. Using a compressor set to provide about 3 psi connected to this rubber device where the air cleaner was attached, a mist of soapy water was sprayed at the carb. The seal between the carb and the offset intake manifold is weak, but if the carb is shoved on as far at it will go, it will seal. The carb gas line brass fitting leaks around the edges but not bad. The drain plug and the idle adjustment screw leak, but this could be sealed with nylon tape. As part of all of this the problem was revealed and appears to be cured as demonstrated on a cold spring ride today. The rubber boots where the throttle cable and a choke cable; fuel enrichment on this carb, must have the rubber boots installed and in good condition. These rubber boots prevent a significant manifold vacuum leak which greatly impacts the performance of the engine. The manual with the engine does not point out the importance of these rubber boots. Checking the status of these seals should be part of engine troubleshooting guides in my opinion. By using the 3 psi pressure test as part of a engine check, including checking the throttle cable and choke cable boots, manifold vacuum leaks could be eliminated and verified before the rider hits the road. This low air pressure did not seem to simply pass through the two cycle engine affecting the test. Other carbs will present a different situation; but these rubber boots may also be a critical seal on other builds. Good luck with your carb problem & happy riding.
  3. Neufcruz

    Neufcruz Member

    I've been having same issue for a while now. I'm running a dellorto clone. Dullar carb. I forget if it had these boots but for sure it doesn't now. Had issues with throttle cable length so the end of cable goes directly into carb. Just sort of sitting there. No mettle screw in fitting or anything. I wonder if this could be my issue.
  4. Greg58

    Greg58 Member

    Timothy you may need to jet the carb, try raising the choke lever slightly as you accelerate. If the jet is too small doing so will make it run better, if it doesn't try a test ride with the air filter removed. If it runs better with the filter removed the jet is too big. Having the carb jetted correctly does wonders for performance.
    Russian2Ride likes this.
  5. Timothy lucas

    Timothy lucas New Member

    After careful inspection I found a small pin hole in the outside cylinder wall over the exhaust port. Looking closer I filled the bolt homes with gas with a finger over the other hole would not hold gas.. I filled the holes with a hardening compound and baked it for 10 minutes at 500°... fixed it....runs like a scalded flaw in the cylinder.
  6. Greg58

    Greg58 Member

    Good to hear.
  7. Hello Moto!

    Hello Moto! Active Member

    I may have the same problem or something very similar. I thought I had a leaky head gasket so I took the head off resurfaced it a bit and made a new gasket. I put it all back together and I'm almost 100% sure that it is sealed now. One big problem though, I still have small leakage above the exhaust port and below the head. Not to mention I have pretty significant power loss above 15mph. I've yet to disassemble it further to inspect the jug but It won't be easy to spot due to the engine being black. I thought a black engine would be cool but I am regretting it now lol.
  8. sully1617

    sully1617 Guest

    well now i think you hit on something. i have the same grubee and have similar problems with the exception that mine won't idle. it runs like hell though and recently the head gasket started leaking due to running it so hard. not sure of top speed but a couple years ago i had a buddy of mine drive his care next to me and he told me i hit 42mph. he has a bike also and mod ed it out and now passes me, probably doing 45mph. anyway i'm gonna take a closer look at those boots and seal them with blue rtv. thanks for the tip