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Pembleton Guzzi 'hopper


Tom Rae

Before I had even bought my Pembleton, Moto Guzzi was part of my plan/dream, I am a sucker for the look of a V twin out front. I then acquired a part built Grasshopper with a 2CV engine and still my dream was for a Guzzi. Reality slowly dawned on me in the form of MSVA and DVLA and I decided that the easiest way to jump these hurdles and get the car registered was to stick with the 2CV. I am sure it was the right decision as I had no problems with either.

Over the next year in use I grew fond of the good old 2CV lump despite its incontinence. It would cruise easily at 70 plus for hours on end and I even uprated it to high compression pistons and barrels, don't laugh, it made a worthwhile difference, it then cruised at 80 and got there quicker too and I did do 11,500 miles in 18 months and it did not miss a beat. But, in my eyes, it still just did not look right.

Well, on the (P)embleton run with Duncan (Silver Surfer) in August 2011 we inevitably chatted about Pembleton matters and, after a few ales, he let slip he had bought a 750 Guzzi for his new limper build but had decided to stick with BMW. I pounced and a deal was done.

It took a bit of time to arrange collection etc and the engine sat in my workshop for a wee bit as the 2CV was still running so well. I slowly started to look at what I had bought, heads off - clean as a whistle, someone had beaten me to it! It had been decoked and valves ground in. This got me on a roll.

How could I fit it? All that was needed was three adaptors and thanks to David Tocher and Dpaws from the forum, CAD drawings were produced. I then started to look for unsuspecting engineers who could produce these. I was in luck, a fellow mountain biker does water jet cutting,( he did my exhaust guards), and he sometimes works with an engineer who had all the right kit. Then, through the forum, I found five others who signed up for the adapter set too thus making a batch of six which was worthwhile setting up for. Quotes were sought from another engineer and they were surprisingly close. I pressed the button and in December I had the six sets in my sticky paws.

After the run to Tan Hill on January 8th 2012, I took the car off the road though it was running better than ever. I removed the 2CV engine, it even had stopped leaking but it was too little too late and its fate was sealed. I did question my own sanity at this point (along with a few others) but the 750 winked at me from the corner and I re-read the stats; weight, same as 2CV; power 48bhp as opposed to 29bhp, bring it on …

I first fitted the flywheel adaptor to the Guzzi engine, I used M8 high tensile bolts with Schnoor washers and Loctite, torqued up to 30nm, next was the adapter for the 2CV flywheel, I had to counter bore some of these for clearance and I even remembered to fit the oil seal in front of the bush. Again, this was torqued up to 30nm with Schnoor washers and loctite. The flywheel would not fit straightaway and had to be lapped in to make an interference fit, this is on purpose. The lapping in was not difficult and soon it was on and ready to be bolted up, once again 30nm, Schnoor and Loctite.

Next was the gearbox adaptor, this required some easy modification (more counter bores) for the bolts holding the adapter to the engine. Torqued up to 10nm with Schnoor washers and Loctite.

Next step is to cut the nose off the gearbox shaft, it is the knobbly bit at the front about 10mm iirc. Then comes the big test, offering it up! I had actually done some trial fits before fitting the adapters and it all went together fine.

For fitting to the gearbox, the crankcase cooling fins on the lower fixings do require to be carefully ground away to make clearance for the nuts, otherwise it is quite straightforward. I decided to use nuts and bolts for this batch rather than trust threaded aluminium as on the original drawing.

However, three adapters do not a conversion make! My plan was to get the engine into the car first and then sort the ancillaries. The few changes to the adapters have been forwarded to the guy who did the original Rev 1 CAD drawings and the ones on the website have been updated. That said, the changes were small, even I could do them, and the engine was then fitted. It looked really good in position, as if it was made for it!

On the 2CV engine, it took a fair amount of patience to set up the Kehin's properly, I always felt uneasy about the angle they were working at, despite the float modification, and adjustment was awkward too. I had decided on new carbs anyway, Dellorto PHBH 30's which are standard for the 750 and I ordered them up from Eurocarbs. Chatting with Dave Parr, he asked me if I had specified stronger float valves to cope with a pressure system doh! I hadn't; but a quick phone call to Eurocarbs and they exchanged my unused float valves for the right ones and at no charge I then simply reversed the Guzzi inlet manifolds and used fuel resistant silicone tubing for the joins and 45 degree bends. This enabled me to position the carbs out of harms way and level with easy access to the tuning screws. Looking on ebay since, I note there are some rubber Honda carb mountings for a 600cc Hornet or CBF 500 with a similar angle and the right inside diameter which might be more rigid. For air filters, I used K&N filters but then I found some much cheaper ones at Gutsibits that would have done the same job, grrrrrrrr.

Jetting proved a bit hit or miss. First I tried with the standard jets as supplied. The main jets for the PHBH carbs are 6mm (be careful as Dellortos also have the same jet size but with a 5mm thread) my 105 main jet was too restrictive, with a big flat spot on hard acceleration, the situation dramatically improved when the choke was pulled out. After some trial and error I settled on a 130 main jet which does seem large but the K&N filters could be partly responsible.

Carbs need fuel, I decided to use the lowest pressure Facet pump (Part no FEP 04SV), recommended by others on the forum. This was mounted under the battery tray (thanks for the idea Peter G) and I fitted an isolation switch under the bonnet. I used the redundant power lead from the 2CV rectifier for this as it is ignition activated. The pump feeds a Filter King filter and pressure regulator and then to the carbs. I have not fitted a return pipe principally because I would have to have extra fuel lines going right back to the fuel tank filler neck but I may do this if I have flooding problems but, so far, it is fine.

My Guzzi engine came bare, no points, no charging system no carbs, in fact not much at all but, what was there was good. I then started to try and source the right kit, I made a few mistakes along the way, (see the for sale ad on the forum) but I then found out that my engine was fitted with a Ducati alternator. This is easily ascertained as it has a parallel crank with a woodruff key instead of a taper , what is not so easy is finding one! I had a stroke of luck on ebay and found a NOS (new old stock) one from a Ducati dealer and they accepted my offer which was actually less than I had been quoted for the stator alone, very happy bunny, hope it fits! Yes, it did fit. What I did not realise at the time is that there was a spacer and O ring behind the rotor and there is a special washer and nut to retain it!

Next problem was the rectifier/regulator, again ebay came to the rescue and a local motor cycle dismantler had a Ducati 600 regulator. I checked the wiring colours and click, it was mine for £35. I did not want wires all over the place so I fabricated a small shelf to take the regulator amongst other things.

This is the shelf I made to take ancillaries:

Erm the right hand reducer on the breather has not been fitted yet.

These pics also shows the jumbo breather hose which goes to my catch tank and foo foo. The rectifier/regulator is on the left under the plate and more in the airflow to aid cooling. The Lucas Rita AB 11 is on the right and the filter King is on the left.

The wiring was a bit bamboozling so I sought help through the forum and Martinclan kindly modified a 2 CV wiring diagram and what an excellent help it was too, ten minutes and the wiring was done! The wiring diagram (below) may only be suitable for my set up and is courtesy of Martinclan.

Next stage was the ignition, there was none on my engine and anyway the sender acted on the Guzzi flywheel which was no longer fitted! Again I was lucky on ebay and found a Lucas Rita (AB 11 is the correct one) and bid on it and suddenly it was withdrawn, oh bu**er missed that one. Then two months later lo and behold it re-appeared from the same seller, I bid on it again and won it for £35, seems to be my lucky number! The Rita came with a Triumph sender which was no use to me however a call to John Carpenter of Mistral engineering soon provided the correct reluctor and plate together with comprehensive instructions on how to fit it. The Lucas Rita should work fine on the standard 2CVDucellier coil and the ECAS Harley Davidson coils, mine is on the latter but they are similar impedance.

Then I had to do the timing. I made a pointer on the side of the gearbox adapter plate

To find TDC (top dead centre) on the compression stroke, I used a digital micrometer in the spark plug hole, it would be helpful to have an assistant for this, that way you can ensure the micrometer doesn't move! l then marked TDC. I then affixed a timing gauge to the crank nose and moved it to 38 degrees BTDC as instructed by Mistral and made a second mark. This mark (not shown in the photo but it is there) is required if you wish to use a strobe to check the timing, it is the full advance and requires the engine to be run at 6000 revs. As Dave Parr says, time for a phonebook down your trousers…

Then I had to set the reluctor/pick up. This was easy, the plate supplied fitted exactly and Mistral also supplied a useful little 5mm spacer for this purpose but I lost it! Once it was set, a quick burl round and check the second cylinder. All good.

Ah, now the exhausts. This caused some angst initially! I found it very difficult to find a pipe bender and fabricator and I was quoted funny money. In actual fact it was easily sorted in the end, I bought two J bends and two 15 degree angles in stainless steel (38mm OD) both 1.5x Diameter which is quite sharp but I think they fit nicely.

I also asked a Guzzi breaker for some retaining collets (new these cost more than £8 each and four are required) and I also asked them for just the neck from two gash exhausts, total cost £6 posted.

It was much easier to find a good stainless steel welder, I have a friend who is one, and after a wee bit of cutting and juggling, he fixed the J bends to the collars and then the 15 degree bends to match up with Pem Mo Co standard exhaust system. We also fitted a balance pipe (25mm OD). I am very pleased with the result.

I originally fitted the exhausts using stainless dome headed nuts with Schnoor washers however they worked loose quite quickly on the first decent run and I have now fitted the correct Guzzi nuts which are rather pricey but they should at least do the job properly.

Crankcase breathing. I understand from NBS motorcycles, Guzzi specialists, that the breathing of a small block Guzzi is less critical than a 2CV. In fact I have blocked off the sump return, Guzzi themselves did this in later models, I have vented the rocker breathers into a catch tank fitted with a foo foo simply because I already had this on my 2CV set up but I suspect it is overkill. Apparently Claus in Germany blocks off all the breathers in his big block conversion.

Lastly, I did some bodywork mods, I extended my nose (stop sniggering at the back) to suit the engine better, I have already mentioned the shelf under the nose, this now houses the coil, the Lucas Rita, the regulator/rectifier, the Filter King, the horn, and it provides a route for the breathers and the control cables for the throttle and choke , busy place, oh and remember to earth it!

How much? I stopped keeping receipts but, all in, the engine swap has probably cost me about £1,000. Savings could easily be made, for example using donor or second hand carbs and alternator and by reading this and perhaps not making some of the mistakes that I did - anyone want to buy some Bing carbs? I could sell my 2CV engine and carbs to help recover some of the cost too. Insurance premium (Adrian Flux) went up by about £10 for the remaining six months of my policy (+£25 for making a mid term correction).

DVLA, I submitted the changed V5 document along with an invoice stating year and engine number and an accompanying letter:

I enclose my amended V5c together with a copy of the receipt for the engine. The dating of the engine has been cross checked with "Moto Guzzi Twins Restoration" ISBN 0 7603 1986 by Mick Walker : Appendix 7 page 22 which supports 1994. I also understand that Moto Guzzi ceased production of this 750 in 1996.

Mike Meakin kindly supplied the dating reference which has also been used by others for VOSA. At the time of writing, no reply so far.

Ah, what's it like then? Awesome in my humble opinion, not macho fast but satisfyingly brisk and entirely in keeping with the chassis and handling, sounds and looks good too. I reckon the power increase is in the region of 65% with no weight penalty which is "better than a slater up your nose" as a friend once said.

The only other related mod on my car is 100% uprated springs (standard shocks), these seem to have kept the compliant handling and improved road holding.

What next? Enjoy what I have!

This is purely an account of my experience, I do not claim to know all, or indeed any, of the answers, far from it, but I can thoroughly recommend the experience. I hope this will be of help and inspiration to someone somewhere. The batch of adapters I had made was a favour from friends and I have already been asked if I can produce more but I am afraid the answer is no however, the amended CAD drawing for the adapters are now on the Pembleton Forum website. The exhaust header I have was a one of, also by a friend, but this should now be easier to source locally by using the stock bends.

At the time of writing, Phil Gregory of Pembleton Motor Company is looking at getting a batch of exhaust headers made.

I don't know if Phil, or perhaps Claus, is interested in producing another batch of small block adapter plates but it is worth asking.

Contacts I found useful - and without whom it simply would not have happened!

  • Pembleton forum; many thanks to forum members, invaluable informed advice and support but in particular thanks to;
  • Mike Meakin, for answering my endless dumb questions
  • Dave Parr, for telling me how to do it properly
  • Alan Walker, for sharing his experience and advice, he's done it already
  • Rob Jenkins for the Guzzi manual and also his first hand experience
  • David Tocher, for the dimensions and additional advice, he is also building one.
  • Dpaws and Creative Genius for the CAD drawings
  • Robin Martin, for help with wiring, he is also building one.

Engineering bits sources

  • Mistral engineering, contact; John Carpenter 020 85 01 21 61 maker of the successor to the Lucas Rita, MOIRA, all round nice guy and a great help, check out the parent Hoppybikes site too.
  • OZJ engineering, bend supplier
    I used:
    • 2 x 180 degree Mandrel J bends 38mm x 1.5D
    • 2 x 15 degree Madrel bends 38mm x 1.5D
  • Eurocarb, Dellorto spares contact; Matt Cooper
  • SCB Motorcycle dismantlers (ebay)
  • Bolt up, stainless fasteners of all sorts (ebay)
  • ASH silicone hose ( have website but ebay prices are better)
  • Gutsibits Ed Wostenholme 01484 841 395
  • And of course, good old ebay!

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