DR750 pilot jets

Questions regarding the DR750

DR750 pilot jets

Postby ian » Tue Jan 09, 2018 1:42 pm

I am busy sorting out a set of Funduro carbs to use with my DR750SJ. What size pilot jets should I use? 40?

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Re: DR750 pilot jets

Postby BigBob » Tue Jan 09, 2018 3:06 pm

They #15 on the SJ and #40 on the 800SL
You could try #40 and 1 and 1/2 turns out on the pilot screws as a start.
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Re: DR750 pilot jets

Postby ian » Tue Jan 09, 2018 3:33 pm

Thanks Bigbob. That was what I was thinking.

But please educate me here. Is the #15 (#17.5 in my book) the size? It seems unlikely because that would seem like a huge jump between the SR41 and the SR42 with a 40 pilot jet? Same airbox and a marginal increase in engine size. Also it looks like those pilot jets are only available in sizes 30 through to 60 in increments of 2.5. So it would be interesting to know what the actual size is. Somewhere around 37.5 to 40 would be my guess? Maybe the pressed in jets have a different numbering system?
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Re: DR750 pilot jets

Postby BigBob » Tue Jan 09, 2018 5:26 pm

The number denotes the diameter in mm so #40 is 0.40mm diameter.
I hear what you say about the jump in size between the 41 and 42 despite only 52cc difference in capacity.
The 41 pilot screws are 2 and 1/2 turns out compared to 1 and 1/2 turns on the 42. Also the needle jet (emulsion tube) is a different size on the 42.
Also, the 42 has a smaller main jet which seems counterintuitive, but it all seems to work.
If you think the differences are quite large between the 41 and 42, you should see the carb specs on the 43 which has the same engine as the 42, but massively different settings.
Carburation is a black art for sure and anything we do is pure guesswork quite often. Most of the time the OE settings are best.
Why are you using Funduro carbs anyway?
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Re: DR750 pilot jets

Postby ian » Tue Jan 09, 2018 8:05 pm

I think the differences for the 43 can be explained by the completely different and much less restrictive airbox. Other than the apparently massive difference in the pilot jet, the differences between the 41 and 42 seem quite minor and the main jet would have no effect on the pilots anyway. It would be interesting to have some more info come to light. But at this stage I am unconvinced that the 41 has a 0.15mm pilot jet. Or if it does, maybe the jet or the metering orifice is somehow different so that it allows more fuel through?

I bought the Funduro carbs a little while back when I had the opportunity and intended to just use them for spares. But I am currently having issues with the original carbs. The bike stood for a while and when I started it, it would not idle, so I suspected the pilot jets were blocked. When I opened up the carbs I found some light deposits of very fine powder like rust in the carb bowls which I suspect came from the tank. I cleaned the tank out and have now completely stripped and cleaned the carbs numerous times, using all methods known to man, but with the same result. The bike starts instantly once carbs are installed, but after a few starts it will only start again with full throttle. I suspect there is dirt or rust trapped in the passageways of the pilot circuit. A clean and blowing with compressed air clears the jet but because the jet is pressed in it is impossible to clear the debris from the passageway. So the bike fires up the first time but shortly thereafter the pilot jet gets blocked again. Anyway that's my theory! We will see tomorrow if I get the Funduro carbs set up.

Another question though. The Mikuni pilot jets available at the local bike shop look identical to the originals but they all have 6 bleed holes which the originals don't have. Are they correct or are they for a different carb?
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Re: DR750 pilot jets

Postby BigBob » Tue Jan 09, 2018 8:28 pm

You are right about the pilot jet being 17.5, I was just going from memory. It`s still very small though, when most carbs use between 30 and 45.
As far as I know the pilot jets have six holes in the side.
Are you sure your fuel pump is working OK and the vacuum hose is operating correctly?
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Re: DR750 pilot jets

Postby dave#22 » Wed Jan 10, 2018 10:52 am

Hi Ian, I've got four DR 750 based bikes running STD carbs, two with K&N filters and two STD airbox with a foam filters..all with aftermarket exhausts...read noisey.

# 1 LH carb 50 idle 130 main.
RH carb 40 idle 125 main.

# 2 LH carb 45 idle 130 main.
RH carb 40 idle 120 main.

# 3 LH carb 50 idle 140 main.
RH carb 40 idle 130 main.

#4 LH carb 45 idle 115 main.
RH carb 45 idle 120main.

as you can see, they all differ...you say your bike won't idle when warm?...have you checked your valve clearance? if it's too tight you won't get a good idle...I set mine to the later .004"IN .006"OUT ...Good luck, Dave.
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Re: DR750 pilot jets

Postby nug » Wed Jan 10, 2018 11:29 pm

Interesting to see that you have "staggered" the jets on all of them Dave. I imagine that its a case of splitting the difference and by doing this you hit the sweet spot in the fuelling. Many hours spent swapping jets I imagine :tup:
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Re: DR750 pilot jets

Postby ian » Thu Jan 11, 2018 8:47 am

Thanks for the info Dave. I can just imagine the amount of time and fun that was to sort the jetting out on 4 bikes! Just with one bike I've had enough practise that from a standing start I can have the seat, tanks and carbs off in 15 minutes!

Anyway, my beautifully logical theory about dirt accumulation trapped behind the pressed in pilot jet seems to be incorrect! I rebuilt the Funduro carbs yesterday with the good internals from my DR carbs and used the Funduros 41.3 pilot jets. Started easily and idled reasonably without any tuning. Fitted the seat and tanks and covers a bit later and then it was back to the same issues. Now will only start on full throttle, won't idle, and running very rich!

The first start up I did just using a gravity fed fuel line to test things before fitting tanks etc. So today I will check to see whether the needles and seats are perhaps sealing ok with the gravity feed, but possibly are not sealing fully with the extra pressure once the pump is connected.
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Re: DR750 pilot jets

Postby ian » Thu Jan 11, 2018 5:14 pm

Ok so todays tests confirmed that the needles and seats are sealing and that the float levels are correct. If the fuel tap is turned off the bike suddenly settles into a nice idle once fuel bowls are almost half empty! The choke seems to be sealing so I'm now out of new theories. I don't want to adjust the floats to this level as the float level is not the issue.
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