I stepped in it - XS750/896 build

Frenchtom

New Member
Well - I finally pulled the trigger after months (years?) of lurking here and other sites. Found a 1977 XS750 that came with 2 XS850 engines and the nice standard tank, not the teardrop style. The engine in the bike supposedly runs, the one on the floor does not but seems like it would with only some select parts added back. What you cannot see are the three or four storage bins full of parts behind where I was standing when taking the picture. The PO chopped the frame which made getting a title a 4 month affair. Just got it this week after buying the bike in September and now its titled as a 2017 custom. Kinda weird but oh well. My plans are a cafe racer style with more emphasis on the racer part and less so on the cafe part. Other details:

Convert the engine to an 896 big bore
Maybe do a cam upgrade
Weld on a hoop and fab a nice teardrop rear fender
Fab a seat
Maybe rearsets although I do not want to lose kick start capability. I may need it with the high compression 896 kit, the starter may not do the job.
No pods!
Powder coat the frame and other select components
Relocate the upper rear shock mounts to a more forward leaning position
New shocks and front springs, progressive rate
Digital ignition module in place of the points and condenser type
Build a new exhaust system (current pipes are chopped up MAC's and probably not salvageable
Tires, brakes, maybe clutch plates, fix second gear, LED brake and signal lights, etc
Paint, buff, polish, the works.

I live about 1.5 hours from Virginia International Raceway and have a stretch goal of running in an old fart's division where passing happens only on the straights and everyone values their real job.

The first advice I would like to ask for is what kind of welding system should I get for the exhaust fab and the shock mount relocation? I have single phase 110, and 220 access and no experience whatsoever.

Thanks,
Frenchtom
 

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Frenchtom

New Member
Well - I spent an hour going through 3 bins of parts and I am pretty sure that I have everything but it is all covered in grease and oil. I found a bunch of '73 Monte Carlo parts from the PO's project car and a bunch of extra XS parts that I will have no use for. The headers that came with the bike appear to be a MAC system that was modified by an angry beaver.

I ordered the 896 piston kit last night and have a co-worker lined up who can bore out the cylinders which I will pull off later today. I am pretty sure the 896 kit comes from china so I am going to measure the piston ODs and grind all the piston down to match the one that is the smallest OD. I'll probably skim the smallest one also to make sure these things are round and not egg shaped. Hopefully I do not open up any porosity. The bike will likely be an 890 CC or smaller when I am done but at least my cylinder walls will be a skoach thicker as a result.

Also found the three Mikuni carburetors which look a little rough on the outside. Will open those up ASAP as well and thoroughly cleanse and rebuild.

Frenchtom
 

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Integra99

Active Member
Welcome Tom, I love the XS750 having built mine into a cafe racer... she is more cafe than racer though!

If you get hold of an 850 loom from 81 onwards this has electronic ignition and 2,000 miles on and had no issues for me, it has a fairly hefty CDI box but I managed to relocate under the seat.. this was temporary but has been ok.

I had trouble with the old Mikinis so fitted a set of Triumph Daytona carbs which worked really well,

Engine is back out for a rebuild currently and to fit some FZR rear sets so phase 2 of my build. She is a heavy old lump so don't get to excited about racing, a guy nick name JCW however has had some good results.. he is on Caferacer.net .

I used FZR600 forks and R6 Calipers on the front.. stops well. She isnt fast but I have fast bikes so I just love the sound of the triple and large intake carbs. It gets a lot of attention but its shaft drive and does suffer a little with handling .. no idea the effects on this with the rear shock angle changed so you may want to investigate this.. I plan to fit some progressive rear shocks later this year.

You have a lot of work so good luck with it all.. engine is fun to get out!

Heres my baby...
107242-220118065655.jpeg
 

Pete12

Been Around the Block
Frenchtom said:
I ordered the 896 piston kit last night and have a co-worker lined up who can bore out the cylinders which I will pull off later today. I am pretty sure the 896 kit comes from china so I am going to measure the piston ODs and grind all the piston down to match the one that is the smallest OD. I'll probably skim the smallest one also to make sure these things are round and not egg shaped.

Frenchtom

Not sure if you're aware or not, but pistons, while being round at the top, are oval shaped in the skirt, with the large diameter on the thrust axis and the smaller diameter on the pin axis. Also, as a result of manufacturing tolerances, pistons for the same application can vary in size, so the cylinders should be bored for each individual piston and marked accordingly for reassembly.
 

XS750AU

Coast to Coast

Frenchtom

New Member
Hi guys and thanks. Both the engines I have are 850cc so it looks like I got real lucky there. For some reason the PO used the triple points ignition system from a 750 in the running 850. I plan to replace that with a Newtronics digital system.

I did not know 1977 had hypo-eutectic (sp?) or oval pistons. Will definitely bore each cylinder to size if that's the case or get the pistons turned by on off-set lathe setup to duplicate the different diameters but with a skilled machinist on the lathe, not some hack. I do not trust ebay chinese stuff so I am concerned about mismatched diameters from piston to piston or inconsistent diameters along the length of each piston.

I like the black engine and silver frame on the Integra bike. I think I will leave my engine unpainted but maybe clear coat it. For frame color I want to do something other than black but nothing radical like yellow. I'm thinking a flat charcoal or dark silver but it will be a long time before I must decide.

Tom
 

XS750AU

Coast to Coast
I do not trust ebay chinese stuff so I am concerned about mismatched diameters from piston to piston or inconsistent diameters along the length of each piston.
So which pistons are you using?
I was a bit skeptical about the "Cruising Image" pistons, but the experienced machinist who bored out the cylinders said they were better dimensionally than the originals would have been. Technology has vastly improved from the 70's. As you can see in the photo the pistons as measured across the skirts (pistons diameter is different top to bottom and gudgeon to skirt. Bottom skirt is max dimension) were within 0.01mm. If there is any doubt then the machinist should bore to suit the individual pistons!

I'm thinking a flat charcoal or dark silver
Yep, I have gone with "Gunmetal grey" with a clear coat.
I will be interested to see how you go with the Newtronics ignition and if there are any advantages over the OEM CDI.
 

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Frenchtom

New Member
I got some more work accomplished. The first order of business was cleaning the 4 basket of parts that came with the bike, they were all covered in a layer of oily slime. In the process I found LED lights that the PO looked like he was going to mount under the frame which made me skeptical of other things I would find.....and I was right. I also took the carbs apart and found the first one to be exceptionally clean and in good shape. The next two, not so much. Full of white scale, crud and in the middle carb, a collapsed float. I noticed that the clip position on the needles were set one groove away from the richest setting likely because of running open pipes and the PO philosophy that more gas = more power.

Now here is a dumb question - I keep reading that 3 into one exhausts and high quality air filters like K&N require re-jetting the bike. The thing is, I cannot find any jets on these carburetors. Is it just the needle adjustment for me?

Then I went to the engine and discovered that when the PO was missing a bolt or gasket he would just keep on assembling without them. For gaskets he used orange silicone, for the missing bolts, he just left them out and filled the holes with silicone. Nice! The bike came with a spare engine so he really has no excuse.

The cam journals were full of rust so now I am thinking the engine sat outside for an extended period of time. The head looks funny too. The right exhaust chamber was brownish colored, particularly the exhaust valve. The middle cylinder was black and the right cylinder was sludged up. There were also what looked like crankcase breather tube running to the engine but only the right cylinder was connected. The middle and left were plugged and may explain the lack of sludge. Two out of 3 exhaust valves failed to seal against paint thinner that I poured into the exhaust port so I think I will lap all six valves for starters and if that does not work I'll cut new seats. I also have a mental note to check for bent or pitted valve stems and worn out guides.

I pulled the cylinders and found minor scoring on the front or exhaust port facing wall, same with the pistons. These will get corrected via the 896 kit.

I have decided that I am going to pull the crank to check bearing clearances and clean out the case. I'll also address the second gear issue these bikes have while I am in there.

Tom

PS - I cannot figure out how to put text directly over or under my pictures.
 

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Pete12

Been Around the Block
Hi Tom, the pilot and main jets are in the carbie bowls - the pilot is accessed from the inside and the main from the outside (underneath). It's not a bad idea as I guess you could change the mains without pulling the carbies off, but that can be verified by someone that owns one.
 

XS750AU

Coast to Coast
Hi Tom
Pete is correct, please refer to attached slides - it appears you unfortunately have Hitachi carbies. Unfortunate as it is supposed to be difficult to get spare parts and jets. As you have probably read most people get hold of a set of Triumph flat slide carbies which are very, very easy to convert over to operate on the XS triple engines. There are 2 brands of flatslides used on the triumph 900 triples, the first preference is the Kehin (Data Dave is using these)and the least favorite are the Mikunis (which I am using). The Mikunis are prone to wear.
Kehins appear to be more difficult to source as people keep hold of them!!!
Good luck.
 

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datadavid

Over 1,000 Posts
Yea, the keihins dont seem to wear out, easy to tune, plus they are the same carbs as on my thruxton so same parts everywhere. Somewhat expensive but can be found in bike breaker's. Mikunis suposedly had a problem with wearing slides and needle jets so triumph changed over to keihin.
 

XS750AU

Coast to Coast
Found a 1977 XS750
Looked at your carby photos again and I think you actually have Mikuni MK1 from the original 750, which are not much better than the Hitachi's. Same advice applies, get a set of Keihin Flatslides or failing that, Mikuni Flatslides (you can get parts to rebuild).
Cheers
 

2_DONE_THE_TON

Over 1,000 Posts
DTT BOTM WINNER
I´ve had the same stock XS 750 carbies and switched to T300 Triumph Triple carbs from a Daytona 900.
Easy to clean, tune and no problems till now whatsover.
You only need to build yourself a choke lever and shorten the stock gas bowden.
 

teazer

Over 10,000 Posts
DTT BOTM WINNER
Tom, the pistons are not only oval but they are also tapered bottom to top. The skirts cannot be machined to size in a lathe. And the barrels cannot be successfully be bored on a mill or lathe. They need to be done by an automotive machine shop with a rigid boring bar and correct hone.

Find a good automotive or motorcycle machine shop and take the new pistons and barrels and have them do what needs to be done. You could also send them to someone like Bill Bune in Anoka MN.

The carbs you have are Mikuni and are easy to work on. Main jets are in the bottom of the bowls under the drain bolt and the pilot jets are in the top of the bowl. There's a good article on the same design carbs at http://pinkpossum.com/GT750/carb/BS40carbs.htm

There are three types of Mikuni pilot jets and some BS series carbs use different ones. MikeXS can supply the correct ones.

The three "vents" on top of the intakes look like vacuum ports to synch the carbs. The one that has a hose on it was probably connected to the fuel tap to create the vacuum to open it.
 

Frenchtom

New Member
Hi Guys and thanks for the replies. I have been out of town and unable to respond until now. I was thinking of rebuilding the old Mikuni carburetors until I found these via google. Are these the correct Mikuni flat slides? They are the 38mm TM series. They have my attention and I like the idea of parts availability.

I came home to fresh pistons sitting in an unopened mail bag and have to say I am quite impressed with their craftsmanship. I took some measurements and while no 2 are the same they are pretty close according to my calipers.

I will try to remove the engine from the frame and split the case halves this weekend. Stay tuned...
 

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XS750AU

Coast to Coast
Hi Tom
Nanno (If you search you will find his blog on this site) used 3 x Mikuni TM carbies to work on his XS750 sidecar. I think they were 36mm, but it was a lot of work, a real lot of work!! You have to figure out how you are going to get 3 carbie cables going into 1 throttle cable. And then you have the issue of 3 individual chokes - how do you activate them. You can only reach 1 of the 3 choke plungers. It has been done but Nanno has put his TM carbies on the market and is now using others that are more simple.
If you get a set from a Triumph 900 triple they basically fit straight in.

The Cruising Image pistons are meant to be made in Japan not China, but who really knows, they are what they are. As long as you have them and give them to the shop that will bore the cylinders on a proper rigid set up there will be no problems.
 

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