Running like $h!t after 88cc kit install

18 replies to this topic
  • blackm20i

Posted June 09, 2009 - 10:01 PM


I have a huge flat spot on my 2003 XR70 after installing the Trail Bikes 88cc big bore kit. Does anyone have any suggestions on how to tune it, im not too good at it. Also I have a high volume oil pump but I have no idea where it is supposed to go!?

I'm pretty sure the timing is right because it will run and go pretty fast but there is just a huge flat spot and at high speeds it doesnt run well at all. Any advice would be appreciated. Thanks.

  • gator9999

Posted June 10, 2009 - 07:51 AM


So when you installed the big bore kit, did you address the carb, I know they recommend their carb to go with the kit or you can rejet you exsisting one

  • blackm20i

Posted June 10, 2009 - 08:22 AM


I bought the new carb that they suggested... do I still need to jet that one?

  • gator9999

Posted June 10, 2009 - 10:58 AM


I wouldn't think so, should be jetted for the kit, whats your plug look like white, chocolate, black that should tell you what direction to go

  • big_perm

Posted June 10, 2009 - 12:59 PM


Just put a 88 in my boys 50 used stock carb work ok but I do need to jet it to get more power. the oil pump is not hard to put in I did that too you will need a clutch tool to get that done and you need to drill out the oil passage to a 2mm hole do a search on the oil pump in this fourm and you will get good instructions thats what I did

  • blackm20i

Posted June 10, 2009 - 01:23 PM


I have the stock spark plug for the 2003 XR70 should I be using something different?

  • Mcduuuugal808

Posted June 12, 2009 - 02:45 AM


You cam may be 180 degrees off, would cause this exact symptom. When cam gear and fllywheel are lined up, cam lobes should face downward.

  • qwoke-q

Posted June 12, 2009 - 05:54 AM


There is no such thing as 180 off. The ignition sparks every crank rotation.

  • Mcduuuugal808

Posted June 12, 2009 - 12:19 PM


There is no such thing as 180 off. The ignition sparks every crank rotation.

see step 27


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  • Towlieee

Posted June 12, 2009 - 12:31 PM


The only thing the cam being 180 out will do, is affect where you set the cam when you adjust the valves :thumbsup:

If the cam is 180 out, you must have it SET 180out when you adjust your valves, or your valves wont be set right (since you'll be setting them on the intake stroke instead of the compression stroke)

To the thread creator, to install the oil pump, you have to take the top end off as well as the clutch.. The top end has to come off to drill out the oil hole bigger. The oil pump isn't 100% required, but it is good peace of mind.

And pull the carb off and check what jetting is in it, I assume a 89 main jet? And did you purchase the race head, or are you using a oem xr70 head?

If your using the oem head, you might need a smaller main jet. But since it's a hesitation at mid range, your needle alone might give you enough adjustment. Easiest way to play with needle settings is to pop the seat off and the gas tank bolts off, that way you can lift the gank, pop the carb cap off, adjust needle, and fire it up and test it.. Then adjust needle as needed..

If no needle settings fix your problem, then you need to find out what your main jet is, and whether your lean or rich

  • xrmarty

Posted June 12, 2009 - 02:15 PM


Will the bike even run AT ALL with the cam off by 180 degrees????? I would think you would have more problems than just a mid range flat spot. Someone fill me in, pls.

As Towlieee already asked, what head are you running OEM or TB??? What size carb?

  • Towlieee

Posted June 12, 2009 - 02:33 PM


Will the bike even run AT ALL with the cam off by 180 degrees????? I would think you would have more problems than just a mid range flat spot. Someone fill me in, pls.

yeah it'll run perfectly fine 180 out, mine is still 180 out to this day :thumbsup:

Just if you adjust the valves while they are 180 out, then you will make them too tight, and the bike wont want to idle right, and will be low on power

But just make sure you have the cam set 180 from where it should be for valve adjustment, and no problems :lol:

If I ever sold my bike someone would be in for a surprise lol :lol:

  • blackm20i

Posted June 12, 2009 - 05:48 PM


Ok thank you for the suggestions I will check my carb tonight. I have the race head by the way and the spark plug is white.

  • shredmasterz

Posted June 14, 2009 - 11:21 AM


I had a similar problem with my 88cc kit with the Chen Wey carb. The needle was way off. You can make it better by running the carb needle in the lowest notch which will raise the needle. Then try a larger main jet. I got the Chen Wey close but then tried a mikuni and had better luck.

  • gator9999

Posted June 14, 2009 - 08:56 PM


White means your running lean, look at the carb directions if you have them to see which way to turn the fuel/pilot screw, I think you turn it in but I'm not sure 1/8th turn at a time, usually adlust it while its running and when it sounds good go for a short ride and recheck your plug. Your looking for a nice chocolate color. cracking and popping on decel is also a lean indicator

  • s_10onbagz

Posted June 14, 2009 - 11:59 PM


how does the bike idle? does it pop when you let off the throttle from being wide open.

  • blackm20i

Posted June 16, 2009 - 11:07 PM


no it doesnt backfire when i let off

  • shredmasterz

Posted June 17, 2009 - 08:26 AM


Unscrew the top of the carb, take the slide off the throttle cable, remove the needle and move the "E" clip to a lower slot position. This will raise the needle and make the air fuel ratio richer. The reassemble in reverse order.

If this does not make the bike run better you may have a over rich condition to begin with and then you should try to move the clip up from the original positon to make it leaner( less fuel).

  • blackm20i

Posted June 22, 2009 - 01:50 PM


Ok I just moved it down from the original position and made it richer and it seemed to really be bogging out a lot. Guess I will try to lean it out

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