Help from an expert needed!

Questions regarding the DR750

Help from an expert needed!

Postby CapnNogrow » Mon Jun 11, 2018 8:48 pm

I have a DR 750 with an 800 engine. Carbs from the DR800, SR43. Cam is from the DR750 ( because I couldnt get an original cam. I also have a Lazer exhaust (Race version). Original unmodifed airbox.
Original carb jetting 45/130 needle 3rd. Runs decent. But a tad too rich. Most likely due to the more open exhaust and the DR750 cam.

So my question is, whats the best approach?

1. Modify the airbox to allow a higher airflow.
2. Put 120-125 (alt 127,5) main jets and fiddle with the needle, keeping the original airfilter and box.

What's your opinion?
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Re: Help from an expert needed!

Postby BigBob » Tue Jun 12, 2018 8:23 am

Your float heights are probably too high so try bending the float tang in towards the needle a smidge. After years of bouncing around they tend to go out of spec eventually.
You could also try a 125 mj if that doesn`t work. Dropping a needle is equivalent to half a jet size.
I`ve tried modifying airboxes without much success as I`ve had the same problem as you, and found it was float height that was the problem.
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Re: Help from an expert needed!

Postby ladder106 » Sun Jul 01, 2018 3:37 am

To begin, I don’t consider myself an expert...just someone who has used a 750 with an 800 engine. The 750 carbs use much smaller 17.5 pilot jets but also have different pilot air jets in the carb mouth. Reducing the pilot jet size should help with richness. I strongly urge you to make certain your carbs and jets are in goo to perfect shape since jetting a dirty carb or one with worn jets is head banging and the time it takes to remove and reinstall the tank and carbs is enough to make sure your changes are effective. Our bikes are old and any big single is very hard on carb slides and needles.

There should not be so much difference between the cams to warrant any major jetting changes

Your more open exhaust will create a leaner condition rather than a richer one and the effect will be more pronounced at higher rpm and large throttle openings.

With the more restrictive airbox of the SR41/42, I’d hazard a guess that your rich condition is most noticed at idle and midrange with maybe some rather loud popping on overrun.

The first thing to do with these carbs after making sure all o-rings are in good shape is to take a close look at the grey plastic carb slides, make sure there are no shiny worn spots in the needles and the needle jets do not have the slightest hint of having a ovalized bore.

The jets are still available from Suzuki or Mikuni jet sites. Chasing jetting when the midrange keeps changing due to a worn needle or worn needle jet is a study in frustration.

My initial advice would be to leave the main jet alone unless the bike won’t pull past 5000 rpm in top due to richness.

I’d start by reducing the pilots down to a 30 or so and drop the needle by raising the clip one notch toward the dull (top) end of the needle. Adjust the idle mix with the screws while engine is warm and idling (not easy without s right-angle screwdriver) and give that a try.

You’ve got a lot of changes all going on at once so expect a few weeks of trial and error before you are happy.

Note that when changing pilot jets you want to replace with jets that do not have the small emulsion holes.

Good luck and let us know how you get on.
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