YZF 750/1000 CONVERSION.

You will have read about this in every magazine, normally followed by the phone number of a man who could do it for you at a reasonable price.

Investigating the possibility, talking to people who have done it and to the builders, It soon became clear that the end result was not an ‘EXUP’ in a 750 frame, but that most of these specials ended up as a peaky thousand with more midrange than the 750 but not as much as the standard 1000. I wanted the feel of an EXUP (pulling from 1000 revs to the red line in top gear) and the handling of the 750 with its adjustability.

Firstly, The routes I did not take.

750 BOTTOM END WITH 1000 BARRELS AND HEAD.  

The advantage is more gears to keep the motor at peak power. But, there is a difference in oil pressure between the 750 and 1000 (according to the manuals) and it might not be a coincidence that several owners had experienced oil related problems. (Big end/cams).

750 WITH BIG BORE KIT.

These kits will give you midrange as any big bore kit will. There was also the offer of a ride in - ride out service. I could not obtain piston weights to compare with standard. The sales talk was an article in a magazine. ( Fast Bikes I think). There was lots of horse power and midrange with extensive work performed to obtain this. However after sifting through the facts it was admitted that the bottom end was not all it could be and would not be like an EXUP. The conversion along with the additional tuning work was expensive.

MIX AND MATCH.

Because the motors are so similar, I encountered a whole lot of mixes of components. 1000 barrels with 750 heads, 750 with 1000 head etc. including a stroked and bored machine using Yamaha components from other bikes. None of the above really appealed to me. My last EXUP was over 40,000 miles when traded and all the ‘750’ solutions had no guarantee of longevity. So I decided it had to be a 1000! Much searching produced a French FZR1000 RU 10,000 miles slight cosmetic, no sorry, written off. This was bought along with ancillaries for £1150. The idea was to have a 1000 Exup set up from air box to silencer. This should give me the Exup power curve I want.

BEFORE FITTING THE ENGINE.

If using the 750 radiator you will also need to use the 750 oil filter housing. (or suffer some really ugly plumbing!). The thermostat and it’s pipe work also need to be changed. The 1000 has the habit of ‘boiling’ the battery and as the 750 utilises a sealed battery requiring accurate charging regimes, I replaced the 1000 alternator with the 750 one. The starter motor was also swapped as the electrical plugs were different. This also has the effect of making the 1000 motor look like a 750!

FITTING THE ENGINE.

Crankcase dimensions are identical on the two motors. The main differences are the oil sight window position and the plumbing. The cylinder head mounts vary with the 1000 having one turret and the 750 two. The engines when bolted in are pulled to the left hand side (for chain alignment) with the right hand side being ‘floating’. This creates a problem as there is no room for a mounting plate to connect the head to the frame holes on the left and only 4mm available on the right. After investigating stepped brackets, it was decided the solution was to remove metal from the 1000 head. The mounting holes are tapped to quite a depth so 6mm was removed from both sides. (6mm and 8mm was the thickness’ of aluminium available to me).

The engine was then put in the frame and cardboard mounting plates were made. After checking and re-checking, I had these copied in aluminium. The plates were then bolted (using tensile bolts) to the cylinder head in the correct alignment for the frame holes. (This took ages as I did not want to loctite and finally tighten the bolts until I was sure they were correctly placed). Once in position it is almost impossible to get a spanner to the bolts. A solution to this is to drill a hole in the frame and then the bolts can be screwed in from the outside when the engine is in situ. I wanted my YZF to look standard so I did not drill holes. With the engine in the frame and the plates in position the final mounting bolts are fitted. Left hand ones first and then the floating right hand ones. The engine is a very tight fit and it may be necessary to taper the left hand plate to clear the castings on the head. The sides of the mounting plates were thickly painted. If there was any flexing of the plates, cracks would appear in the paint.

There is plastic cover on the 750 which shields the coils and seals onto the rocker cover. This was cut down the fit the taller 1000. Everything else should just bolt on as normal. The radiator can go on it’s standard mounts provided some metal is removed from the fan motor’s supporting frame.

EXHAUSTS.

The exhaust manifold (with Exup valve) from the 1000 was used to ensure tuned lengths were kept standard for the motor. There are differences in the muffler end of the 1000/750 pipes. Some Exup pipes require a gasket, some are too short to meet the silencer. I started using the 750 silencer and this works satisfactorily although it is heavy. The Exup valve connects to the servo the same on all bikes. The standard thousand silencer will not mount to the footrest hanger without some fabrication. I now run a Micron carbon fibre race pipe.

CARBURETORS.

These are the same bore (38mm) on both bikes but have differing internals on some circuits.  (See table 1) Imported bikes may have different diameters (i.e. smaller diameter) I ended up using the 750 carburetors as I could sell the 1000 ones and need money.

AIR BOX AND FILTER.

Most 1000 YZF's I had seen had dispensed with the air box, and used ‘ramair’ type filters. These will flow huge quantities of air and I would expect top end is exacerbated. Unfortunately, there are no bottom end gains with these filters, in fact the lack of an air box causes confusion below 3000 rpm. 

Where the YZF air box is used (and it will fit under the tank) the filtered area is too small to flow the air required by the thousand. (See calculations below) The designs of the air boxes are very different. The 1000 uses a long tapered intake into one chamber (the center of the filter). There is a large internal volume and short rubber bellmouths on the carburetors. The YZF uses the underside of the tank as a chamber, a short parallel sided intake (for noise problems I think, try your bike with it removed!) leads to another chamber of which the filter forms one side. Directly behind the filter is the bellmouths which are longer than the 1000’s.

AIR BOX HOLE. 

With rubber            30mm X 75mm = 2750mm2                 25mm X 110mm = 2750mm2

Without rubber       36mm X 86mm = 2952mm2                                                                

Hole is big enough. Removing rubber gives greater area but probably reduces airflow smoothness.

FILTER

(filter area calculated by measuring height of filter material, no of folds and depth.)

                          90mm X 1480mm = 133200mm2                    90mm X 2080mm = 187200mm2

750 area is 29% less than 1000 area.

AIR FLOW.

Calculated at the red line for each engine, should give the maximum volume of air that goes through the filter.

750

13500 rpm = 225 r p sec on 4 cylinders

as each cylinder draws in air sequentially, airflow can be calculated on one cylinder.

2 revs = I cycle

1 cycle = 187.5 cc air

therefore 112.5 X 187.5 = 21093.75cc air per second at 13500rpm

1000

11500 rpm = 191.67 r p sec on 4 cylinders

1 cycle = 250.5 cc air

therefore 95.835 X 250.5 = 24006.6675cc air per second at 11500rpm

This demonstrates that the 1000 motor requires just under 14% more air at the red line than the 750.

The following calculation shows the 1000 motor will struggle to breath at around 10000 rpm due to the restriction of the air box filter.

21093.75/250.5 = 84.2066      84.2066 X 2 = 168.4132 rp/sec which is 10100 rpm approx.

This result was experienced on the road.

Running with the standard YZF air box limits the power and on the road it was noticeable that the bike stopped accelerating just above 9000 rpm. When compared to a 1990 Exup, the roll on in top gear was the same up to 8500 rpm then the standard bike disappears into the distance. Flat out acceleration through the gear from a standing start really shows the lack of ability to rev. Running the standard YZF air box with out the filter allows the engine to red line but causes carburetion problems in the bottom and midrange areas.

The 750 air box was examined to find ways of increasing the filtered area, but there was no (easy) solution available. The air box from a 1000 was cut up but would not fit either. Finally a Thunderace  air box was used. The dimensions are similar to the 750 air box but the filter is an oval type. The box fits in the hole and even the mounting screw lines up! It is slightly too tall on the corners and rather than jack the tank up to clear it (about 12mm), I decided to mould the lid of the box to fit with the tank in the standard position. This was done by hack-sawing wedges out and using a heat gun, bending them down and welding them together. It’s best to practice on an old air box first or get a professional plastic welder to do it. The important thing is to keep the curves, as they guide the air into the bellmouths. The carburetors are different on the Thunderace compared to Exup. As is common practice on a lot of new bikes, the pilot air circuits vent to the air box rather than the inside of the bellmouths. This meant that the holes in the air box were too large for the carburetors I had.

Again cardboard and putty came to the rescue. A former was made and repeated fittings of the tank over the air box enabled me to find the correct position for the carburetors. The cut off bottom of an Exup air box was used as a pattern to make a steel plate (no aluminium was available) which locates in the Thunderace air box and is held in by screws and bolts. I had to use the 750 bellmouths as the ones I bought from the breakers with the motor were the wrong size (French carburetors!!) The whole affair was sealed with a combination of silicon sealant and araldite. It is important to think about the location of any screws used and glue them so they don’t find their way into the engine.

IGNITION.

A comparison of the diagrams for the two bikes showed the wiring to be similar, with differences on the starting/cut out circuits. The actual ignition curves were very different! The ignition box also controls the Exup valve. Only one wire was removed from the loom and the ignition box fitted in (it uses the same plug but you will need an RU box as the earlier ones had two plugs and differed in connections.

SPROCKETS.

The YZF chain and sprockets were used. The chain size is the same and there is negligible effect on gearing. The 1000 uses smaller sprockets to obtain the same gearing. It is possible there is a chain run clearance problem if you ran the smaller sprockets.

RESULTS AND CONCLUSION (AS THEY SAY).

The end result of my tinkering was 1000 air box, 750 bellmouths, 750 carbs,1000 motor (standard), 1000 exhaust and a race pipe. Run with 1000 ignition. Cooling by 750 radiator.

The bike was recently dyno'd and produced 124 Bhp at the rear wheel. This is average for an Exup. The main thing was the linear delivery, which was spot on. There is a slight glitch at 1000 to 2000 rpm which is not a flat spot, but noticeable (well nobody else has noticed it!). The bike has now done 6000 miles and I need to do the valve clearances. I should have done them before fitting the motor. There is ample power and I can catch most bikes I’ve chased, but it may be possible to improve the package. I intend to investigate the relationship between Rpm, air box volume and length of bellmouths. I will also source some Exup carburetors to replace the 750 ones. This should remove the glitch. After that there is a Dynojet kit although the dyno operator said I would gain no improvement with fitting this!

Weight distribution seems OK, weight saving work is ongoing. The bathroom scales showed no difference in weight, but they are not calibrated!

Dismantling the two heads shows the 750 to be engineered to be lighter with hollow cams and waisted parts. Valves are smaller to give less reciprocating weight. The 750 cams have a higher lift than the 1000 and could fit in the 1000 motor. I did not weigh the two engines but the 1000 could be slightly heavier due to these small but numerous differences.

Overall, the bike is a whole lot easier to ride than the 750 and more flickable than the 1000 Exup

Table 1 - 1000/750 Comparison.

Once again, many thanks to Andy Baxter for putting together this report, but I have chickened out and just gone and bought an YZF-R1 after all that!!!   Still worth doing if your on a tighter budget though.

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Last updated October, 2005

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