Fitting carbs to XJ750D (22n) injected

Hello,

I have a 83 XJ750D that was getting a bit tired @ 129,000 km so I brought a 24L engine complete n cheap. It came with carbs. Has anyone done the modification to carbs and what would be required in changing from injection regarding the computer/wire loom?

Thanks for any help.

See also my next post on injection problems.

 

I had a buddy that did the reverse with a VFR and there were differences with the R/R. The Fi model would work in the carbed one but not vice versa. That was all I remember for now, sorry.

 

I recall mention of this conversion somewhere in the past. Hit our search function and see what you can find.

 

Hello Robert,

I have had a look but cannot find any info for this modification. I did find a member who has a bike that was modified when he brought it, AdeXJ. It is my backup plan if I cannot sort the injection problems ;-)

Thanks.

 

Hi Kiwi750D,

Had the reverse situation - 750D carbed - looked around for Fuel Injection to put back on it, but was advised against it (especially as parts are non-existant now).

As far as I know, my 750D has had no mods to wiring loom (I have a connector that isn't used from the TCI). The Instrument panel works ok - but the Fuel economy, Range, and Consumption does not work as that is calculated via the Fuel Injection wiring (so I been told).

Have had quite a time with the carbs (especially after the first clean), with airbox flooding, too rich, too lean - and its mainly trial and error.

I have taken out the oxygen sensor in the airbox as it was restricting airflow, changed the petcock on the tank to a vacuum operated model from a Kawa GT750 (kept forgetting to turn it off and flooding the engine when stopped).

When running it is responsive 1-5k rpms, with a flat spot between 5-6K, and then very responsive from 6-redline.

I have yet to try and move the carb needles up/down, but that is my next job - hopefully that will eliminate the flat spot.

If you have changed the engine to a 24L, do the carbs match up to the airbox - if running rich, the oxygen sensor may be blocking the air intake.

Hope you get it sorted.

 

Hi AdeXJ,

Thank you so much for that.

I was looking at the wire loom and was hopeing that no mods needed to be made. Shame the trip information stopped working.

I have the manual so can check the jet sizes for the carbs. I am running a standard exhaust so I hope mixtures will be right.

Thanks for the tip about the petcock!

I noticed that the timing rotor is different on the carb motor, I will try both once I get things sorted and let you know if it makes a difference.

The carbs I have do not look like the needle can be adjusted.

The carbs I got with the 24l are Hitachi and I guess they are the correct ones for that motor, they match up to the airbox.

Yeah, I hope I can sort it out, really like to find out wtf happened to the injection. Bike ran really well untill I took it apart!

Cheers!

 

Which manual are you going from - I'm using the XJ650&750 Fours manual (1980-1984) - seems to cover most things ok.

My carb jets were 92.5's and working fine when I got it - unfortunately, I forgot the golden rule (If it ain't broke, don't fix it!!) and took most of the bike apart, cleaned it, replaced it - minor problems ever since.

After the clean, it was running lean, so I increased the Jets to 112.5's (nearest I could get to stock) and then it ran rich, so I removed the oxygen sensor and it seemed to run fine. (My bike has 4-1 Motad, so jetting may be different for yours)

I currently have a problem with flooding, so the carbs have got to come out again to recheck the floats, and I will be adjusting the needles at the same time - if you take the top off the carbs and remove the rubber diaphragm (careful, they are old), there may be a clip inside in the middle of the diaphragm which holds the needle in place. This would also be a good time to make sure there are no splits in them.

Haven't got a clue about the timing rotor - I don't usually do much with the engine (see golden rule above!!).

Good luck.

 

Hi AdeXJ

Same manual you have!

The carbs are dirty so I am cleaning them today. The rubber diaphragms are ok, one return spring is missing. Will have a look at the needles and write here if they are adjustable.

Yes mine worked fine the motor was just going to cost too much to do so I pulled everything apart put the new engine in and now the injection does not work correctly! When I get it all working ether way I will post here and will let you all know how I got on and if that rotor makes any difference.

Thanks to everyone here, very helpful and a fantastic site that I will be visiting often!

Good luck too you too with the flat spot!

 

Hi Kiwi,

I had success today - Carbs came out, cleaned and found one float that was broken and therefore not shutting fuel off. Replacement found at local shop (free if I stripped it down myself).

Moved all the needle stops down a notch (increase flow) and the bike now has no flat spot that I could find (mind you, 6000rpms in 2nd is about 60mph, so I didn't get a chance to try 3rd or higher - local police crackdown on speeding!!!), so hopefully it is ok - at least there is no more fuel on my garage floor.

Hope you have as successful a time with yours.

 

Hello Ade,

It is good that you have fixed the problem, I did not see any adjustment on my carbs needles so it will be the washer method for me.

I cleaned my carbs, the Hitachis were dirty but worked ok. The springs had been stretched to a third again of their length, be interesting to see if they work ok.

I unplugged the EFI stuff and took the airfllow sensor out of the airbox, plugged up the 3 holes created by removing it and removed the boost sensor. Made up an adaptor for the choke cable as it was too long. Turned on the fuel choked the carbs and hit the starter, vrooommmm, it lives. Have not taken it on the road yet job for tomorrow.

If all ok (she is not flooding now) I will tune it using the DIY tools on this site and a colour tune if I can find one locally. I have been told that the carbs have more go above 6000 rpm than EFI so that is cool. Economy is not as good though, oh well!

Yes the cops don't like too much speed here either. A local club has meetings at Levels Raceway in Timaru so I will leave it till then to find out what she will do!

Thanks again!

 

I am glad you got it running, we did not get many of the FI XJ bikes over here, from what I have seen. Great work!

You will have to combine your posts together for our technical section, so we can have a DIY carb-FI page. Then you will be famous and chicks will dig you.

 

Great news that it is running again.

Mine is VERY responsive from 6k-redline, almost like a turbo kicking in.
Fuel consumption is not too bad if you keep it in the 3-6k range - mine averages around 8-10km/liter (35-45mpg) in this range, which is better than most bikes I've had before (My XJ900F does about 25-35mpg).

Top speed currently achieved so far was 180 kmh (about 110mph) and that was about 7k rpm in top gear.

 

Just for information, how much space is there to get your carbs in/out.

Mine are a pig to remove and replace as they are almost touching the starter motor underneath, and the air box only moves about 10mm back - makes for a very cramped space (btw XJ600 carbs match the rubber boot positions on a 750 Engine/airbox)

 

@ spinalator

Will do when I get time maybe tonight.
Oh and by the way chicks already dig me ;-)

@ Ade

Don't know what my top speed was I got it to 160km but the fines here are rather more than I'm willing to pay! With EFI the power was smooth and very responsive but you don't get sudden increases like with carbs.

The trip computer told me 21Km liter @ 100km/h on the open road normal cruising. At 95Km/h slip streaming a truck the computer reported 33Km/l. Dave said a well tuned carb 750 should do 17Km/l normal cruising @ 100Km/h.

I am going to take the bike for a spin tomorrow if the weather holds. One spring was missing from the carb (in the slide) and I have not been able to find an exact replacement, carb 1 has a spring the same diameter and length but with only half the coils so will get on the phone and try to get a new one.

There is plenty of room for the carbs but the bottom of the petcock hit the choke rail and stopped the choke working. I had to dremil off a bit of the rail and place a thick washer under the rear tank mounting to get enough clearance.

 

33 km/l = 92mpg.
Don't think I've ever had a bike that's done that well on fuel (apart from my old 50cc MZ when I was 16 - happy, cheap, and slow days)

17km/l = 48mpg.
That sounds about right - I think I got my calculations wrong on the previous post!!! - I hate trying to convert between kmh, km/l and mph, mpg.

I have to guess the average fuel consumption based on miles done and how much it costs to fill up.

Are you still using the FI petcock, or did you get a vacuum operated tap?
If using a vacuum tap, did you connect the hose to the carb itself, or to the rubber boot.
Not sure if it makes a difference on which one, but the 900 has the vac tube going to the boot of cylinder 3 (inside right when sitting on it), but the 750 has it going to the carb of cylinder 2.

Hope the weather holds out for you.

 

Hi Ade

At 33Km/l I was 10 feet behind the truck (naughty, gets you a fine if caught) and was being pulled along, you get close enough and you can feel it. I had a full tank and the gauge showed 1.5l used I filled back up and it took 1.5l so 33Km/l was spot on.

Yes still using the EFI tap but going to change it. No place on the carbs for a vac feed, will take it off No.2 intake. No.1 is still connected to the boost sensor and I still have the temp sensor on the head connected. I disconnected the throttle position switch.

Weather held, went for a ride. Not running the best (lean up top) so I am re-cleaning the carbs and double checking the float levels. The parts are soaking now and will go over it with the compressor later. Will also pull the plugs and have a look, plugs can tell you so much! Have not had much luck finding a slide spring. Local shop is seeing if they can get a new one for me. If they do I will ride down and replace the spring on the side of the road, getting really fast at tearing the bike down. Less than 10 minutes to get the carbs out!

Hope to get a new tap tomorrow and have found a oil cooler too. Next job a fresh coat of paint. Will be taking a lot of photos to and will post them to the gallery here.

 

Kiwi, if you can't get a spring let me know, I have one I can part with off a Hitachi carb.

 

Hello Nick,

Thank you for the very kind offer. If I am stuck I will let you know.

 

So how is the Carbbie running now?

 

Have been busy...but with the carbs the bike ran lean and backfired, did much searching on the net and found references to exhaust leaks so today I took everything apart and I could see that the exhaust seals on the head are leaking (could see burnt carbon deposits) so had a look at the other head and the seals are nice and copper coloured.
OK I had the ones on the bike tight so I took the seals out and took the old ones out too...the ones in the new engine are only half as thick! They could not seal the exhaust. Will try and get some Monday (as they get destroyed removing them) and report back here if the problem has been resolved!
I have attempted every thing else apart from the TCI so it looks like four stupid little copper rings!

 

Got new exhaust gaskets, crossed fingers and hit the starter button. runs great!
Been riding round last couple of days and have got heaps of power, in fact the more I ride the better the engine goes (according to the trip computer the economy is increasing), I have it back to stock running EFI.
Now for the bad news, number 3 is down on compression (120 psi) and the oil control ring is not controlling the oil.
It does not affect the power, when I tuck down under the windshield and listen I cannot hear any "bad noises" but it is using too much oil.
When the shops are open again I will order a gasket set and fix it. Am thinking about swapping my good top end (barrels and pistons & rings) off my old engine with the good bottom end I have now.